Auto-cleanup properties

This is a proposal for having properties doing some cleanup after a property execution. The implementation is not so pretty (`unsafePerformIO`) but this reduces the burden of having to bracket code with `Temporary.using`.
I don't have a strong opinion about this. On one hand it'd be good to know that some clean-up is going on and at the same time when I read the spec I don't want to be distracted by this boilerplate stuff.