Redirect and/or gag stdout/stderr.
Documentation (with examples): https://stebalien.github.io/gag-rs/gag/
TODO:
General:
* Windows support.
* Better error handling?
Redirect:
* Be generic over references. That is, accept both a reference to an AsRawFd or
an AsRawFd. Unfortunately, I don't know if this is even possible. Borrow
doesn't work because I really want the following constraint:
`impl<F> Redirect<F> where F: BorrowMut<T>, T: AsMut<AsRawFd>` so I can write
`file.borrow_mut().as_mut()` but that would be ambiguous...
Buffer:
* Deallocate the buffer as it is read (FALLOC_FL_PUNCH_HOLE) if possible.