Move caller_filter to `rspec/caller_filter`.

- The caller_filter is generic for all of RSpec, not just
  rspec-mocks specific.
- It's also at `rspec/caller_filter` in core (and will be
  in rspec-expectations soon). The idea is that any lib can
  require `rspec/caller_filter` and get this.
- Add spec to verify the file stays up to date.