Fixes for record type typecode caching, and additional tests (attempt 2)

A continuation of the work/discussion from https://github.com/numba/numba/pull/879, using a dict to cache keys instead.
Scratching my head a bit more... Dispatcher_Insert() is also called independently by Dispatcher.compile(). At this point it can't know any values corresponding to the given typecodes, so it doesn't know which keys to use for a typecode cache insertion.