* 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>
