Derive instances for Generic and NFData

I'm able to work around this [like so](https://github.com/reach-sh/reach-lang/compare/63cfe3482bd9...0f9a66aa7875#diff-f985de41a397939bbab675172143b267) but it would be nice if these data structures came with these instances.

If you're interested in this, let me know; I'd be happy to write up the PR for it.