Replace ExternalStream with new ByteStream type (#12774)

# Description
This PR introduces a `ByteStream` type which is a `Read`-able stream of
bytes. Internally, it has an enum over three different byte stream
sources:
```rust
pub enum ByteStreamSource {
    Read(Box<dyn Read + Send + 'static>),
    File(File),
    Child(ChildProcess),
}
```

This is in comparison to the current `RawStream` type, which is an
`Iterator<Item = Vec<u8>>` and has to allocate for each read chunk.

Currently, `PipelineData::ExternalStream` serves a weird dual role where
it is either external command output or a wrapper around `RawStream`.
`ByteStream` makes this distinction more clear (via `ByteStreamSource`)
and replaces `PipelineData::ExternalStream` in this PR:
```rust
pub enum PipelineData {
    Empty,
    Value(Value, Option<PipelineMetadata>),
    ListStream(ListStream, Option<PipelineMetadata>),
    ByteStream(ByteStream, Option<PipelineMetadata>),
}
```

The PR is relatively large, but a decent amount of it is just repetitive
changes.

This PR fixes #7017, fixes #10763, and fixes #12369.

This PR also improves performance when piping external commands. Nushell
should, in most cases, have competitive pipeline throughput compared to,
e.g., bash.
| Command | Before (MB/s) | After (MB/s) | Bash (MB/s) |
| -------------------------------------------------- | -------------:|
------------:| -----------:|
| `throughput \| rg 'x'` | 3059 | 3744 | 3739 |
| `throughput \| nu --testbin relay o> /dev/null` | 3508 | 8087 | 8136 |

# User-Facing Changes
- This is a breaking change for the plugin communication protocol,
because the `ExternalStreamInfo` was replaced with `ByteStreamInfo`.
Plugins now only have to deal with a single input stream, as opposed to
the previous three streams: stdout, stderr, and exit code.
- The output of `describe` has been changed for external/byte streams.
- Temporary breaking change: `bytes starts-with` no longer works with
byte streams. This is to keep the PR smaller, and `bytes ends-with`
already does not work on byte streams.
- If a process core dumped, then instead of having a `Value::Error` in
the `exit_code` column of the output returned from `complete`, it now is
a `Value::Int` with the negation of the signal number.

# After Submitting
- Update docs and book as necessary
- Release notes (e.g., plugin protocol changes)
- Adapt/convert commands to work with byte streams (high priority is
`str length`, `bytes starts-with`, and maybe `bytes ends-with`).
- Refactor the `tee` code, Devyn has already done some work on this.

---------

Co-authored-by: Devyn Cairns <devyn.cairns@gmail.com>

crates/nu-plugin-test-support/src/lib.rs

@@ -82,7 +82,7 @@
 //!     let input = vec![Value::test_string("FooBar")].into_pipeline_data(Span::test_data(), None);
 //!     let output = PluginTest::new("lowercase", LowercasePlugin.into())?
 //!         .eval_with("lowercase", input)?
-//!         .into_value(Span::test_data());
+//!         .into_value(Span::test_data())?;
 //!
 //!     assert_eq!(
 //!         Value::test_list(vec![

crates/nu-plugin-test-support/src/plugin_test.rs

@@ -93,7 +93,7 @@ impl PluginTest {
     ///         "my-command",
     ///         vec![Value::test_int(42)].into_pipeline_data(Span::test_data(), None)
     ///     )?
-    ///     .into_value(Span::test_data());
+    ///     .into_value(Span::test_data())?;
     /// assert_eq!(Value::test_string("42"), result);
     /// # Ok(())
     /// # }
@@ -136,33 +136,44 @@ impl PluginTest {
 
         // Serialize custom values in the input
         let source = self.source.clone();
-        let input = input.map(
-            move |mut value| {
-                let result = PluginCustomValue::serialize_custom_values_in(&mut value)
-                    // Make sure to mark them with the source so they pass correctly, too.
-                    .and_then(|_| PluginCustomValueWithSource::add_source_in(&mut value, &source));
-                match result {
-                    Ok(()) => value,
-                    Err(err) => Value::error(err, value.span()),
-                }
-            },
-            None,
-        )?;
+        let input = if matches!(input, PipelineData::ByteStream(..)) {
+            input
+        } else {
+            input.map(
+                move |mut value| {
+                    let result = PluginCustomValue::serialize_custom_values_in(&mut value)
+                        // Make sure to mark them with the source so they pass correctly, too.
+                        .and_then(|_| {
+                            PluginCustomValueWithSource::add_source_in(&mut value, &source)
+                        });
+                    match result {
+                        Ok(()) => value,
+                        Err(err) => Value::error(err, value.span()),
+                    }
+                },
+                None,
+            )?
+        };
 
         // Eval the block with the input
         let mut stack = Stack::new().capture();
-        eval_block::<WithoutDebug>(&self.engine_state, &mut stack, &block, input)?.map(
-            |mut value| {
-                // Make sure to deserialize custom values
-                let result = PluginCustomValueWithSource::remove_source_in(&mut value)
-                    .and_then(|_| PluginCustomValue::deserialize_custom_values_in(&mut value));
-                match result {
-                    Ok(()) => value,
-                    Err(err) => Value::error(err, value.span()),
-                }
-            },
-            None,
-        )
+        let data = eval_block::<WithoutDebug>(&self.engine_state, &mut stack, &block, input)?;
+        if matches!(data, PipelineData::ByteStream(..)) {
+            Ok(data)
+        } else {
+            data.map(
+                |mut value| {
+                    // Make sure to deserialize custom values
+                    let result = PluginCustomValueWithSource::remove_source_in(&mut value)
+                        .and_then(|_| PluginCustomValue::deserialize_custom_values_in(&mut value));
+                    match result {
+                        Ok(()) => value,
+                        Err(err) => Value::error(err, value.span()),
+                    }
+                },
+                None,
+            )
+        }
     }
 
     /// Evaluate some Nushell source code with the plugin commands in scope.
@@ -176,7 +187,7 @@ impl PluginTest {
     /// # fn test(MyPlugin: impl Plugin + Send + 'static) -> Result<(), ShellError> {
     /// let result = PluginTest::new("my_plugin", MyPlugin.into())?
     ///     .eval("42 | my-command")?
-    ///     .into_value(Span::test_data());
+    ///     .into_value(Span::test_data())?;
     /// assert_eq!(Value::test_string("42"), result);
     /// # Ok(())
     /// # }
@@ -219,7 +230,7 @@ impl PluginTest {
             if let Some(expectation) = &example.result {
                 match self.eval(example.example) {
                     Ok(data) => {
-                        let mut value = data.into_value(Span::test_data());
+                        let mut value = data.into_value(Span::test_data())?;
 
                         // Set all of the spans in the value to test_data() to avoid unnecessary
                         // differences when printing

crates/nu-plugin-test-support/tests/custom_value/mod.rs

@@ -143,7 +143,7 @@ fn test_into_int_from_u32() -> Result<(), ShellError> {
             "into int from u32",
             PipelineData::Value(CustomU32(42).into_value(Span::test_data()), None),
         )?
-        .into_value(Span::test_data());
+        .into_value(Span::test_data())?;
     assert_eq!(Value::test_int(42), result);
     Ok(())
 }

crates/nu-plugin-test-support/tests/hello/mod.rs

@@ -80,7 +80,7 @@ fn test_requiring_nu_cmd_lang_commands() -> Result<(), ShellError> {
 
     let result = PluginTest::new("hello", HelloPlugin.into())?
         .eval("do { let greeting = hello; $greeting }")?
-        .into_value(Span::test_data());
+        .into_value(Span::test_data())?;
 
     assert_eq!(Value::test_string("Hello, World!"), result);
 

crates/nu-plugin-test-support/tests/lowercase/mod.rs

@@ -73,7 +73,7 @@ fn test_lowercase_using_eval_with() -> Result<(), ShellError> {
 
     assert_eq!(
         Value::test_list(vec![Value::test_string("hello world")]),
-        result.into_value(Span::test_data())
+        result.into_value(Span::test_data())?
     );
 
     Ok(())