Add Atomic.compare_exchange

[TheNumbat]

The atomic interface has compare_and_set, which returns a bool indicating whether the operation was successful.

It's often the case that you want to know the previous value upon failure, so this PR adds compare_exchange, which returns the previous value. Its semantics are otherwise the same.

Only adds a simple test, since the implementation is now used by compare_and_set.

[glittershark]

is this missing assembly for the primop in backend/x86...?

[TheNumbat]

This actually doesn't need a backend implementation - compare_and_set also always calls into the runtime so it can check domain_alone/write barrier/etc. The lock_cmpxchg in the x86 backend is only used for particular intrinsics.

We should consider compiling the atomic primitives directly to instructions when their argument is an immediate, though. (In another PR)

[glittershark]

lgtm