Externalize methods and state that was previously held on all objects.

- Remove `@mock_proxy` ivar
- Remove `rspec_verify`, `rspec_reset`, `__mock_proxy` and `__remove_mock_proxy` methods.

This is a refactoring that I'm doing to pave the way for the new expect
syntax (#153). As that syntax "wraps" objects in order to mock or stub
them, we need to externalize these methods and state. The methods directly
on the object (e.g. `stub` and `should_receive`) will simply delegate to
this external logic.

This refactoring had some nice side benefits:

- No need for the hacky YAML fix. It was only needed because we were storing
  `@mock_proxy` on the mocked object.
- AnyInstance no longer needs to much with `dup`; again, this was only needed
  because it dup'd the `@mock_proxy` ivar.
- The Marshal extension gets significantly simpler (again, due to not
  storing `@mock_proxy` on the object anymore).

Rather than storing the mock proxies on the objects, we are now storing
them in a hash, keyed by object_id.  This is pretty simple and consistent,
but could be problematic for objects that muck with `object_id`. That method
seems a bit sacred, though, so I'm not too worried about it.