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-protocol/src/hir.rs

@@ -909,6 +909,7 @@ pub struct Call {
     pub positional: Option<Vec<SpannedExpression>>,
     pub named: Option<NamedArguments>,
     pub span: Span,
+    pub is_last: bool,
 }
 
 impl Call {
@@ -981,6 +982,7 @@ impl Call {
             positional: None,
             named: None,
             span,
+            is_last: false,
         }
     }
 }