Optimize generic implementations of Read and Show

[Lysxia]

I don't think this is going to be a bottleneck for anyone, but the task could be an interesting exercise for someone to get some practice with how GHC rewrites Core.

1. Set up some inspection tests for read and/or show comparing with GHC-derived instances (following some examples for other classes, currently in branch inspect)
2. Optimize so the tests pass
3. (optional) Do some benchmarks to get an idea of what we win