Add equality operations for Reps.

This speeds up microbenchmarks by 50%, and new-metrics.rkt by 30%.

My microbenchmarks show that `type-equal?` is still an order of magnitude faster on non `eq?` values. Two options to persue are to use `type-equal?` everywhere or get it so that `eq?` is enough. I think we should be able to do the second with some work.