Makes our various mutexes available to Signal

[huntc]

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Embassy provides various types of Mutex e.g. Noop. This PR allows the mutex to be provided as per what we do with the MPSC. A major benefit is that the previous situation of needing a critical section can often be avoided.

A consequence of the change is that the new function is no longer const. This means that, as per the MPSC, Forever needs to be utilised e.g.:

static SOME_SIGNAL: Forever<Signal<Noop, SomeCommand>> = Forever::new();

// Then, during initialization...
let some_signal = SOME_SIGNAL.put(Signal::new());

[hydra]

This might need rebasing now that https://github.com/embassy-rs/embassy/pull/792 is merged as it uses a Signal.

[huntc]

Just writing up here what became the blocker: Functions in traits cannot be declared const...

The RFC states:

Traits, trait implementations and their methods cannot be const - this allows us to properly design a constness/CTFE system that interacts well with traits - for more details, see Alternatives.

I'm unsure that there's a way forward here.

[Dirbaio]

Done in #961