Add #![deny(warnings)] to the crate

[alexcrichton]

With lots of platform specific code it's pretty likely that a whole bunch of various warnings crop up here or there. What do you think about trying to reel these in with a blanket denial of all warnings?

[japaric]

What do you think about trying to reel these in with a blanket denial of all warnings?

Or ... we could just set #![allow(warnings)]!

Sounds like a chore. But I'm in favor.

[alexcrichton]

Done!