stabilize atomics (now atomic)

This commit stabilizes the `std::sync::atomics` module, renaming it to
`std::sync::atomic` to match library precedent elsewhere, and tightening
up behavior around incorrect memory ordering annotations.

The vast majority of the module is now `stable`. However, the
`AtomicOption` type has been deprecated, since it is essentially unused
and is not truly a primitive atomic type. It will eventually be replaced
by a higher-level abstraction like MVars.

Due to deprecations, this is a:

[breaking-change]

src/test/compile-fail/std-uncopyable-atomics.rs

@@ -12,7 +12,7 @@
 
 #![feature(globs)]
 
-use std::sync::atomics::*;
+use std::sync::atomic::*;
 use std::ptr;
 
 fn main() {