Proposal: move Mio-pipe into Mio

[Thomasdezeeuw]

The Mio-pipe crate provides a simple wrapper around a Unix pipe. It's currently ~300 lines of code, but I think that can be reduced if it's lives inside the Mio repo, mainly by using IoSource.

One current disadvantage is that it doesn't have Windows support. But @carllerche has been working on that in #1351.

Source: https://github.com/Thomasdezeeuw/mio-pipe
Docs: https://docs.rs/mio-pipe/0.1.1/mio_pipe

[carllerche]

Sounds good to me. I might call the fn pipe() instead of new_pipe(), but that is minor. It can live in mio::unix?