Stream output while subprocess is running

[dimo414]

This PR reimplements Bkt::execute_subprocess() and its callers to support streaming the subprocess' output while it's running, rather than blocking silently until the subprocess finishes.

As suggested in https://stackoverflow.com/q/66060139 the child process' streams are processed in background threads responsible for both persisting and streaming the output to the caller's stdout/stderr.

Scoped threads are used to support streaming to references such as &mut Vec<u8> and to loosen the type requirements of the stream (namely to not require that they are 'static).

CLI:

• Subprocess out/err streams are duplicated to bkt's out/err while the subprocess runs, instead of caching all output first and then writing the cached output after the subprocess completes.
• Streaming is enabled by default and is not configurable.

Library:

• Added Bkt::retrieve_streaming and Bkt::refresh_streaming methods which accept out/err sinks. The subprocess' output is written to these streams in addition to being cached and available in the returned Invocation.
• The preexisting Bkt::retrieve/Bkt::refresh behave as previously, i.e. they do not support streaming.
• These methods are marked "experimental" and are subject to change or even removal.

Fixes #31

[dimo414]

Moving this out of the PR description before merging:

I'm positing this as a PR to solicit feedback before merging, so please feel free to weigh in on any parts of this change you think could be improved. Some areas for potential feedback:

• The signatures and ergonomics of the affected and new methods
  • In particular the impl Write+Send trait bound (which prevents us from using StdoutLock/StderrLock because they are not Send)
• The use of scoped threads to avoid the 'static trait bound
• Error handling and Result propagation
  • Notably, currently the .join() calls are .expect()-ed because they don't seem to be compatible with ?
• The DisregardBrokenPipe behavior, and whether it should be moved into the library core