Add "final" inference

We currently disable some optimizations such as tail recusion elimination and purity inference for _non-final instance methods_ based on the fact that we cannot make assumptions about them, since they can be overriden later by users of haxe-generated code.

While this is a safe bet, I believe it is actually rare when Haxe code is used like that, and it would be more beneficial to actually infer and set `final` for both classes and methods. It would enable both Haxe and target-native optimizations and would still be safe to use from outside (if target language supports checking for `final`). It would also benefit code that does not use inheritance much.

Of course, it should be opt-in, so how about `-D infer-final`?