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/tests/commands/cp.rs

@@ -188,7 +188,7 @@ fn copies_same_file_twice() {
 #[test]
 fn copy_files_using_glob_two_parents_up_using_multiple_dots() {
     Playground::setup("cp_test_9", |dirs, sandbox| {
-        sandbox.within("foo").mkdir("bar").with_files(vec![
+        sandbox.within("foo").within("bar").with_files(vec![
             EmptyFile("jonathan.json"),
             EmptyFile("andres.xml"),
             EmptyFile("yehuda.yaml"),

crates/nu-cli/tests/commands/sum.rs

@@ -32,3 +32,31 @@ fn all() {
         assert_eq!(actual, "448");
     })
 }
+
+#[test]
+fn outputs_zero_with_no_input() {
+    Playground::setup("sum_test_2", |dirs, sandbox| {
+        sandbox.with_files(vec![FileWithContentToBeTrimmed(
+            "meals.json",
+            r#"
+                {
+                    meals: [
+                        {description: "1 large egg", calories: 90},
+                        {description: "1 cup white rice", calories: 250},
+                        {description: "1 tablespoon fish oil", calories: 108}
+                    ]
+                }
+            "#,
+        )]);
+
+        let actual = nu!(
+            cwd: dirs.test(), pipeline(
+            r#"
+                sum
+                | echo $it
+            "#
+        ));
+
+        assert_eq!(actual, "0");
+    })
+}