Loom support

[sistemd]

Would you accept a PR adding a loom feature to the crate, which swaps out std::sync with loom::sync? This would allow people who test their concurrent code with loom to also have loom control the scheduling of dashmap locks.

[xacrimon]

@sistemd I would yes. Gladly!

[shade]

@xacrimon Any issue with using shuttle instead of loom? We would get the same exhaustive testing that loom does but with the ability to use randomized testing if needed.

I am happy to write the PR :)

[l1x]

I have created a basic PR with shuttle:

#370