Add LooseSignatureVerifier

See https://github.com/rspec/rspec-mocks/pull/726
I think `length` is kinda a funny variable name for this...while `args` is an array, I think of it the args as being closer in semantics to a tuple (even though ruby doesn't really have tuples like other languages do) than an array.  What do you think about calling this argument `positional_arg_count`?