Remove const class_info pointer. We don't use it anyway.
Leave class_info userdata on the stack in __newindex as well.

It actually triggers some weird optimization problems, and yields code that
performs worse. But let's pretend it's a compiler problem. Actually on clang
it's not so obvious.
Prehash strings. May help with class hierarchy lookups.
Minor type cleanups.
Make method_info references const. Minor change.