Limitations

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. You can get around this though by using the --nocapture argument when running your tests.

TODO:

General:
- 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.