Add LooseSignatureVerifier

See https://github.com/rspec/rspec-mocks/pull/726
My thinking was that I didn't want to smear this concept across multiple files. The way it is now, the coupling to other files is a nicely symmetric register/deregister. Without this (or moving it elsewhere), you expose register/direct array, which makes me uncomfortable. It's private code anyways, but I still have a slight preference for maintaining cohesion here.

...if that's not convincing I'll change it.