defmethod: validate arg count according to arglists

[markus-wa]

hi @camsaul I attempted to solve #59

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[markus-wa]

maybe for code coverage we could extract a function and use this hack? https://stackoverflow.com/a/37471356 what's your thoughts on that?

[markus-wa]

re testing: I'm not sure how to best do this right now, as we're trying to test a macro that throws an exception, which doesn't seem to be easily testable with standard clojure.test tooling. was hoping you'd be able to guide me to a solution here

[camsaul]

Thank you for taking the time to work on this PR.

I ended up implementing this in #130 -- it ended up being quite a bit more complicated than I originally thought, since we have to be able to handle & rest (vararg) arities.