refactor: define node matchers on assertion classes

[G-Rath]

I figured this would be enough but didn't want to overload #1790.

It does send the fixer for InternalAffairs/NodeMatcherDirective into an inf. loop though - it looks like one part of the cop sees just the "self" whereas the other sees the whole definition, so it reports "use self.match_assertion instead of self" and then the fixer just keeps adding .match_assertion to the end of the string until it errors; for now I've just disabled it, but I might try and look into it at some point.

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[G-Rath]

at this point the names are getting a little silly - I'll next explore if we can nicely have the classes handle calling their match logic and just return a result

[bquorning]

@G-Rath I pushed an alternative implementation at the same time as you opened your PR: #1794. I hadn’t considered using class methods for the node matchers, that’s a neat idea.