Support functions, class methods, and descriptors for custom rules

[jayaddison]

This PR is to handle a fairly specific scenario where a Grammar is defined inside a Python class, and refers to custom rules which are static methods on that class.

In parsimonious==0.8.1, attempting the following will fail:

class ExampleParser():
    @staticmethod
    def custom_rule(text, pos):
        pass

    grammar = Grammar("""
        ...
        """,
        custom_rule=custom_rule
    )

The failure occurs when an exception is raised during creation of the rule map, and the root cause is that the reference to custom_rule is resolved by Python as an unbound method descriptor.

This PR resolves the issue by adding a specific check to see if a rule matches the inspect.ismethoddescriptor check; if it does, we also need to retrieve the __func__ value from the reference in order to obtain a Python callable.

The PR also handles a similar case where a (non-static) class method is bound to a custom rule, but an argument count mismatch occurs due to the presence of the instance self argument in object method calls.

Edits: enable syntax highlighting, fix indenting, make syntactically valid

[jayaddison]

cc @erikrose - I've been experimenting with using custom rules as part of a small ingredient parsing grammar and ran into issues when using class-methods as handlers, which I've attempted to address here. Glad to make any adjustments and modifications; thanks for the library!

[jayaddison]

Small nudge @erikrose - do you have any time for review of this PR? If the change doesn't seem appropriate or there are any issues with it, I'm happy to close or modify it.

[jayaddison]

Thanks for the review @evanh @erikrose! Hopefully mostly set, glad to apply any further cleanups.

[erikrose]

Thanks for the great patch, @jayaddison, and for the great reviews, @evanh! We're finally starting to get things unstuck around here. :-)