libxev's .rearm semantics are unfortunately not very
portable---rescheduling the exact same event on macOS / kqueue seems
to cause the event loop to hang, while manually rescheduling and
returning .disarm does not.
This loop adds an xev.File wrapper called TtyWatcher which delivers
events to the users callback. Note that this implementation does not
handle any of the writes. Writes are always safe in the main thread, so
we let users decide how they will schedule those (buffered writers, xev
writes, etc)
