Factor out code discovery to separate class

[apjanke]

Put the code that does class/method/function discovery in a separate class. That way it'd be easy for doctest to copy it if they want it, and easier to test. Seems like this is going to be a need throughout Testify, since Octave’s methods() and meta.class aren’t currently up to the task of discovering everything we need. (Especially with the case of shadowed duck-punched methods for classes that are defined spread across multiple source directories.)