Add LooseSignatureVerifier

See https://github.com/rspec/rspec-mocks/pull/726
Given the fact that this is only used for tests and `matcher_definitions` can just as easily be accessed from a test, I think I'd prefer to not see this method and instead have the test call `matcher_definitions.delete(block)`.  (Or you can define `deregister_matcher_definition` as a helper method in the test context if you find it helps make the code clearer.)