core: Move unstable to unstable/mod.rs
core: Move locks, atomic rc to unstable::sync
core: Rename SharedMutableState to UnsafeAtomicRcBox
auto merge of #6387 : brson/rust/unstable, r=brson

r? @pcwalton

* Move `SharedMutableState`, `LittleLock`, and `Exclusive` from `core::unstable` to `core::unstable::sync`
* Modernize the `SharedMutableState` interface with methods
* Rename `SharedMutableState` to `UnsafeAtomicRcBox` to match `RcBox`.