Move external closer to internal (#1611)

* Refactor InputStream and affected commands.

First, making `values` private and leaning on the `Stream` implementation makes
consumes of `InputStream` less likely to have to change in the future, if we
change what an `InputStream` is internally.

Second, we're dropping `Option<InputStream>` as the input to pipelines,
internals, and externals. Instead, `InputStream.is_empty` can be used to check
for "emptiness". Empty streams are typically only ever used as the first input
to a pipeline.

* Add run_external internal command.

We want to push external commands closer to internal commands, eventually
eliminating the concept of "external" completely. This means we can consolidate
a couple of things:

- Variable evaluation (for example, `$it`, `$nu`, alias vars)
- Behaviour of whole stream vs per-item external execution

It should also make it easier for us to start introducing argument signatures
for external commands,

* Update run_external.rs

* Update run_external.rs

* Update run_external.rs

* Update run_external.rs

Co-authored-by: Jonathan Turner <jonathandturner@users.noreply.github.com>

crates/nu-cli/src/commands/command.rs

@@ -179,7 +179,7 @@ impl CommandArgs {
             args: T::deserialize(&mut deserializer)?,
             context: RunnableContext {
                 input,
-                commands: registry.clone(),
+                registry: registry.clone(),
                 shell_manager,
                 name: name_tag,
                 host,
@@ -215,7 +215,7 @@ impl CommandArgs {
             args: T::deserialize(&mut deserializer)?,
             context: RunnableContext {
                 input,
-                commands: registry.clone(),
+                registry: registry.clone(),
                 shell_manager,
                 name: name_tag,
                 host,
@@ -238,13 +238,13 @@ pub struct RunnableContext {
     pub shell_manager: ShellManager,
     pub host: Arc<parking_lot::Mutex<Box<dyn Host>>>,
     pub ctrl_c: Arc<AtomicBool>,
-    pub commands: CommandRegistry,
+    pub registry: CommandRegistry,
     pub name: Tag,
 }
 
 impl RunnableContext {
     pub fn get_command(&self, name: &str) -> Option<Arc<Command>> {
-        self.commands.get_command(name)
+        self.registry.get_command(name)
     }
 }
 
@@ -530,7 +530,6 @@ impl Command {
 
         let out = args
             .input
-            .values
             .map(move |x| {
                 let call_info = UnevaluatedCallInfo {
                     args: raw_args.call_info.args.clone(),
@@ -597,7 +596,7 @@ impl WholeStreamCommand for FnFilterCommand {
         let registry: CommandRegistry = registry.clone();
         let func = self.func;
 
-        let result = input.values.map(move |it| {
+        let result = input.map(move |it| {
             let registry = registry.clone();
             let call_info = match call_info.clone().evaluate_with_new_it(&registry, &it) {
                 Err(err) => return OutputStream::from(vec![Err(err)]).values,