[python] get rid of HxBuiltin, clean up things here and there
[python] try to clean up Native(Haxe)Iterator(Iterable) again.
[python] avoid "Void should be Dynamic" in Syntax.foreach
[python] don't wrap classes that inherit from BaseException in _HxException
show proper error message when a class method has different number of arguments than required in implemented interface (closes #3417)