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.rs

@@ -77,6 +77,7 @@ pub(crate) mod rename;
 pub(crate) mod reverse;
 pub(crate) mod rm;
 pub(crate) mod run_alias;
+pub(crate) mod run_external;
 pub(crate) mod save;
 pub(crate) mod shells;
 pub(crate) mod shuffle;
@@ -189,6 +190,7 @@ pub(crate) use reject::Reject;
 pub(crate) use rename::Rename;
 pub(crate) use reverse::Reverse;
 pub(crate) use rm::Remove;
+pub(crate) use run_external::RunExternalCommand;
 pub(crate) use save::Save;
 pub(crate) use shells::Shells;
 pub(crate) use shuffle::Shuffle;