Add a feature to make spin use optional

[pierre-rouanet]

Hi @phil-opp and thanks for your really useful crate!

I'm using it to create an allocator for a Cortex-M0.
Unfortunately the spin Mutex is not supported as it uses the sync AtomicBool which is not defined for this architecture.

So I've forked your repo to add a feature that make spin optional and if it's defined, it enables the LockedHeap. Thus, it is still compatible with other uses such as the one in https://github.com/pollen/alloc-cortex-m for instance where it uses its own Mutex to define a LockedHeap.

Let me know if you would be interested in integrating this.

Cheers!
Pierre

[phil-opp]

Thanks a lot! I didn't think about architectures without atomics when I added LockedHeap. Nice to hear that the crate is useful to you.

[phil-opp]

There is now a version 0.5.0 on crates.io, which includes this PR.