Do precise subtype tests in instanceof

[dcodeIO]

Addresses #2577. One rather trivial part of the issue was that previously, since interfaces are not derived from Object but their implementing classes are, an someInterface as Object instanceof SomeInterface would fail. Another was that the check for when a cast may possibly succeed was overly simplistic, thus required more elaborate infrastructure.

The seemingly obvious alternative that doesn't work is to simply fall back to do dynamic checks if the relationship between the types is arbitrarily complicated (and hence possibly costly to compute), since that breaks assumptions about such checks being precomputable.

Making a draft for now while I untie the knot in my brain before figuring out how to best test the changes. Is perfectly possible that I missed something, hence I'd appreciate any amount of eyes on the new logic :)

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[CountBleck]

I don't really get this stuff, so I can't help much :)

[dcodeIO]

So I wasn't quite happy with the previous take and decided to refactor this more. Now, extendees and implementers are populated early using a bunch of ugly loops that are probably less ugly than frequent recursion, so the checks, whereof there may be many, are just hash map lookups.