Rename arity calculator spec helper method to match new API.
Ruby warns on private attr_reader, even though test_attr seems to define it as acceptable behaviour.
Add regression test for expecting dynamic class methods.
Negative spec for expecting dynamic methods on instance doubles.

This is unconvential stylistically, but I like it better than nothing or
a comment buried in the code.

WDYT @myronmarston ?
Correctly verify expectations when constant is removed.
refactor: Introduce ModuleReference abstraction.
Push exception generation into ErrorGenerator.