Move caller_filter to `rspec/caller_filter`.

Created by: myronmarston

• 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.