[#17] Try getting method signatures directly from doubles.

  Add SimpleDouble.
[#17] Extend rspec double instances with ability to record interactions
  instead of wrapping rspec double instances with SimpleDouble or RecordingDouble
  objects.

  Set VerifiedDouble.registry to RecordedMethodSignatureRegistry.

  Fix: stack frame of recorded method signatures point to library code, instead
    of lowest spec code.
[#17] Clean up codebase.
  Remove RecordingDouble and RelaysToInternalDoubleReturningSelf.
  Remove SimpleDouble::MessageExpectation and SimpleDouble::MethodDouble.
  Remove InstanceDoubleValue because there's only one kind of instance double
    value now (RspecDoubleValue).

  Update Value.from spec: ensure that if content is a class double,
    Value.from(content) will return ClassValue.