stateref: Abstraction for things that work like IORef.

Include instances for STM types

Include instances for TMVar (TMVar not available in hugs' stm implementation). I have been unable to make this flag's value depend on whether or not you're running hugs, so if you are, you'll just have to change it manually. It'd sure be nice if there were either an explicit backtrack command in cabal or if 'buildable:false' would trigger a backtrack. Or if I could say "default: not(impl(hugs))" for this flag. Note that this is irrelevant at the moment because I broke Hugs+STM support by adding MonadIO m => ... instances. If anyone really wants me to fix it, speak up - but those instances are pretty darn convenient.