Extract the allocator into an separate crate

[jessebraham]

New crate's repository can be found here: https://github.com/esp-rs/esp-alloc

Made no real changes to the code which was extracted, only what was necessary to get everything building/running for both chips. I was able to get things working using critical-section==0.2.7 instead of using a git dependency like we had discussed previously.

I did test this on hardware and things seem to be working as expected.

[bjoernQ]

LGTM

One thing that changed is the fact that there will be always a global allocator now - which isn't bad since the Wi-Fi driver uses heap memory anyways

[jessebraham]

That's a great point, we can always change this though if needed!