Redirect and/or gag stdout/stderr.

Documentation (with examples): https://docs.rs/gag/

Limitations

• Doesn't work on windows. Patches welcome.
• Won't work if something else has called std::io::set_print (currently unstable). Unfortunately, this function doesn't actually redirect the stdio file descriptor, it just replaces the std::io::stdout writer.
• Won't work in rust test cases. The rust test cases use std::io::set_print to redirect stdout.

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 (FALLOCFLPUNCH_HOLE) if possible.