Move Io/Pio/Mmio from redox_syscall to common

* Fix Mmio repr from transparent to packed.
* Rename Mmio::from to Mmio::new.
* Remove dead imports around affected code.
* Minor formatting.

ps2d/src/controller.rs

@@ -1,5 +1,5 @@
 use log::{debug, error, info, trace};
-use syscall::io::{Io, Pio, ReadOnly, WriteOnly};
+use common::io::{Io, Pio, ReadOnly, WriteOnly};
 
 use std::fmt;