Remove unmaintained spin crate.

[ghost]

Close #163.

I chose spinning-rs as a replacement for spin as it is actively maintained and requires few changes to the code base.

Another alternative is spinning_top from rust_osdev, however this doesn't implement Once<T>.

[BurntSushi]

I'd like to point out that this appears to add two new dependencies to the tree as well, lock_api and scopeguard. The spin crate had no dependencies.

[ghost]

@BurntSushi We could also write our own Once<T> type, but this would add more to maintain. I will gladly add that implementation if you prefer though.

Edit: lock_api depends on scopeguard, so replacing spin will add another dependency, scopeguard.

[zesterer]

spin-rs is now maintained again.

[ghost]

Thanks.