Move vector_clock and sync into concurrency, make vector_clock private

[RalfJung]

#1963 introduces a concurrency module collecting the data race and weak memory code. If possible, it'd be nice to make vector_clock private to that module, too; that will require moving sync.rs (and maybe thread.rs) into it as well.

[RalfJung]

Reopening to track moving thread.rs into the concurrency module.