Irq-safe locking via Mutex and RwLock.

Offers identical behavior to the regular spin crate's Mutex and RwLock, with the added behavior of holding interrupts for the duration of the Mutex guard.

When the lock guard is dropped (falls out of scope), interrupts are re-enabled if and only if they were enabled when the lock was obtained.

Also provides a interrupt holding feature without locking, if desired. Interrupt safety is achieved simply by disabling interrupts, which currently works for x86 architectures only.