Work in progress (do not land): expose arena as type parameter

[dwrensha]

This uses Generic Associated Types, which are not on stable rust yet. Also, it looks like it's getting pretty complicated, and it's still in very rough draft proof-of-concept form.

It does, however, bring some improvements:

• some benchmarks run ~10% faster, presumably due to elimination of dynamic dispatch
• potentially resolves #201
• allows Alignment to be specified as a type parameter, so we would no longer need the unaligned feature flag
• eliminates some RefCell hacks