Replace Override.simple with Override.create

This pull request:
1) Renames Override.simple to Override.create
2) Adds support for the missing features that the override functions needed (returning longs and using the context).
3) Ports all the override functions to use Override.create
(If we don't end up needing the context at all, then in the future I'd like to move *opts.static* to a single-character flag in the key, as we've done for the method and field caches.)