irq_safety: Interrupt-safe spinlock Mutex/RwLock

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, see the HeldInterrupts type.

This crate is designed for no_std usage within an OS kernel or in an embedded context.

Supported architectures:

• x86
• x86_64
• aarch64
• arm

We welcome contributions from anyone, especially for new architectures.