Cleanup: add support for ad-hoc method check logic

There's an increased demand for supporting method resolution steps featuring different method check logic (other than the standard one based on subtyping). Examples:

-) arity-based check (a method is applicable if the number of arguments 'fits' the method signature)
-) existence check (a method is applicable regardless of actuals/formals)

Currently javac is 'faking' some of those custom logics by relying on error types (as an error types in javac is compatible with anything). It would be nice to have explicit support for ad-hoc resolution checks.