Feature Request: Allow setting the standard streams of Unix.create_process with optional arguments.

While it is convenient that `Unix.create_process` creates default pipes for the user, it would be even *more* convenient if they could optionally be set to something else, in case the user would like to connect them directly to another open file descriptor or the standard streams of the parent process.

I'd be glad to make a PR if that's appropriate.

Start by reading the current Unix.create_process API and how it creates default pipes. Define how optional standard-stream descriptors should interact with existing behavior, then add coverage for supplied descriptors and the parent’s standard streams; done means the new interface works without breaking current callers.

Written by the indexing model from the issue text.