methodize / func_to_method_func: position-only params put self after the / #118

Found in the review of #117 (pre-existing on master): methodizing a function that has position-only params inserts self after the /, so the method signature is wrong (e.g. def f(a, /, b) with methodize(f, bind='b')). Also from that review: a default_from param's signature default is an InstanceAttrDefault marker; code that JSON-serializes signature defaults (front, py2http OpenAPI, cf. #88/#89) should be checked if it ever meets a methodized function.

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