Add Modify constructor to State, and handleState function

[axman6]

I've separated this from #44 as it's probably more controversial. Adding a Modify constructor allows users to know that a modification will be atomic. If you look at the example in the diff, I've shown how this can be used to allow users to run stateful computations on mutable shared state, concurrently with other threads, or isolated in the traditional manner, without modification to the computation.

An alternative implementation would be to implement Get and Put in terms of Modify, but then the example I've provided would end up calling atomicModifyIORef many more times than necessary.

Keen to hear feedback on this, I'm not confident that this change is actually worth it, and am sure there are problems I haven't considered.

[axman6]

Uh, looks like I branched from the wrong branch, so the commits from #44 have been included, but I probably wouldn't want this included without those either. I'm happy to make a new PR with the one commit that differs here if you'd prefer, since they don't actually depend on each other.