Added arch specific mutex implementation

[korken89]

I started the implementation for an arch specific mutex to get discussion going.

[rust-highfive]

r? @therealprof

(rust_highfive has picked a reviewer for you, use r? to override)

[jonas-schievink]

CriticalSectionMutex is not Sync, so it doesn't really work as a mutex like this

[jamesmunns]

Hey @korken89, I'm not sure it's possible to share this mutex between an interrupt and main with this, at least without using unsafe, as your mutex is not Sync.

Additionally, it's kind of a bummer that this mutex panics if the resource is taken, but I guess that was already decided based on the Mutex-Trait API.

[korken89]

Right, I have to move to UnsafeCell

[jonas-schievink]

What happens now if I lock the mutex while already holding it?

[korken89]

Ah right, now I remember the original issue around the implementation - thanks for pointing it out.
I'll take a another look the upcoming days.

[therealprof]

Discussion from the meeting: Seems this has stalled out. Maybe this could be another focus project? If people are interested in joining such a group, please speak up.

The MSP430 team is considering taking over the current implementation from bare-metal to not keep unneeded dependencies around.

[newAM]

I'm triaging Cortex-M issues, I think this has been solved with critical-section. Feel free to re-open if there's still something to discus here!