Module critical

Zephyr implementation of critical sections.

Critical sections from Rust are handled with a single Zephyr spinlock. This doesn’t allow any nesting, but neither does the critical-section crate.

This provides the underlying critical section crate, which is useful for external crates that want this interface. However, it isn’t a particularly hygienic interface to use. For something a bit nicer, please see [sync::SpinMutex].