sendmmsg / recvmmg support

[esemeniuc]

A significant number of rust libraries interface with mio, and not having something like sendmmsg to batch syscalls results in more performance overhead than necessary. I saw #185, and I hope you'd reconsider since mio is so fundamental.

[Thomasdezeeuw]

Mio follows the same API as the standard library. You can use the vector I/O methods ofio::Read and io::Write with Mio, but it will not support {send,recv}mmsg (unless std lib starts to).

Socket2 (a different crate I also maintain) has the SockRef type which can be used with the try_io method on Mio types to perform more advanced I/O.