Add support for static methods in interfaces (#680)

This change allows an interface to include `static` methods as requirements, so that types that conform to the interface will need to satisfy the requirement with a `static` method.

The essence of the check is simple: when checking that a method satisfies a requirement, we enforce that both are `static` or both are non-`static`.

Making that simple change and adding a test change broke a few other places in the compiler that this change tries to fix. The main fix is to handle cases where we might look up an "effectively static" member of a type through an instance, and to make sure that we replace the instance-based lookup with type-based lookup. There was already logic along these lines in `lower-to-ir.cpp`, so this change centralizes it in `check.cpp` where it seems to logically belong.

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