An interruptible stream can query an `AtomicBool. If that bool is true,
the stream will no longer produce any values.
Also introducing the `Interruptible` trait, which extends any `Stream`
with the `interruptible` function, to simplify the construction and
allow chaining.
The extra newline character makes it hard to use nu as part of an
external processing pipeline, since the extra character could taint the
results. For example:
```
$ nu -c 'echo test | xxd'
00000000: 7465 7374 test
```
versus
```
nu -c 'echo test' | xxd
00000000: 7465 7374 0a test.
```
This improves incremental build time when working on what was previously
the root package. For example, previously all plugins would be rebuilt
with a change to `src/commands/classified/external.rs`, but now only
`nu-cli` will have to be rebuilt (and anything that depends on it).
