Flesh out the std::sync::Arc mock implementation

[whentze]

This PR adds the following APIs to loom::sync::Arc<T>:

• Arc::ptr_eq
• Arc::strong_count
• Default
• From<T>

It also adds some basic docs (the current docs just say "TODO").

Unresolved questions:

• Should we add support for weak refcounting?
• Should we add some tests for this Arc?
• How do we implement the methods that std uses fences for (try_unwrap etc.)?

[seanmonstar]

Thanks!

[seanmonstar]

Hm, seems a CI build got lost, there's nothing for me to "restart" it. Could you force push to trigger CI again?

[whentze]

I triggered CI again, but it still gets stuck. I believe this is an instance of travis-ci/#10204, as #12 is stuck at the same check.

[carllerche]

Fun fun... we should migrate to azure pipelines.

[carllerche]

I tried restarting travis. If that doesn't work, I will force the merge.