Implement Release, AcqRel, and SeqCst fences

[tomtomjhj]

This PR implements Release, AcqRel, and SeqCst fences.

• Release fence can be implemented by recording the causality of the thread, which is joined with the later stores (see released: VersionVec).
• AcqRel fence is Acquire fence + Release fence.
• SeqCst fence uses the old implementation of SeqCst, which was removed in Relax how SeqCst is modelled #108 for being too strong. However, that implementation is the correct if SeqCst is used only for fences. Please see Atomics load with SeqCst ordering strange behaviour  #180 (comment) for more information.

Partially addresses #180
cc: @jeehoonkang @wvwwvwwv

[carllerche]

Hey! So sorry we haven't gotten back to you faster. The reality is, I am the primary maintainer here but don't have the bandwidth for ongoing maintenance if it isn't needed by Tokio. That said, it looks like you have expertise in the area. Would you be interested in getting involved on a regular basis and help maintain Loom?

[carllerche]

Thanks. It looks like you know what you are talking about, so I'm going to trust you on this :) As mentioned in an earlier comment, Loom needs more dedicated maintainers and I'm hoping you can help out. Feel free to ping us in the loom channel on https://discord.gg/tokio

[tomtomjhj]

Hi. I probably won't be able to actively engage in the maintenance tasks in the next 2 months, but I can take some time to monitor the issues and review the PRs.

BTW, I added a commit to list the unsupported features I'm aware of.

[Darksonn]

I'll make a release soon.