Use the Loom atomic verification framework in this crate

[notgull]

This pull request resolves #1 by enabling a configuration option that uses the Loom framework.

Note that the current set of Loom tests seem to fail. I don't know enough about atomics to figure out how to fix it? I still say this is a step in the right direction.

[Restioson]

Note that the current set of Loom tests seem to fail.

I believe that one of these is due to running non-loom tests with loom. Loom requires you to use its types within a loom::model block, so only loom tests should be run with loom.

The other one that triggers seems to be the same across multiwrite and upgrade:

Completed in 1 iterations
unsync_mut_at = VersionVec { versions: [0, 1, 0, 0] }

Causality violation: Concurrent load and mut accesses.

thread 'tests::multiwrite' panicked at 'Causality violation: Concurrent load and mut accesses.

This comes from the compare-exchange weak on line 87.

[notgull]

That's odd. Even when I change both operations to SeqCst it still breaks.