Refactor and simplify HasTraits method checks

[mdickinson]

The HasTraits machinery needs function and method checks at various points, mostly to look for magic-named methods like _mytrait_changed and _myevent_fired.

Those checks are currently performed inconsistently, and for methods written in Cython also rely on a rather fragile mechanism for recognising the method. (That mechanism also fails for C extensions other than Cython extensions.)

This PR:

• makes the checks consistent: they all now rely on the _is_unbound_method_type helper function.
• uses inspect.ismethoddescriptor as a better way to catch something that behaves like an unbound method without relying on Cython-specific details
• cleans up unused related functions and constants

[kitchoi]

LGTM