objtuple: Go out of the way to support comparison of subclasses.
Two things are handled here: allow to compare native subtypes of tuple, e.g. namedtuple (TODO: should compare type too, currently compared duck-typedly by content). Secondly, allow user sunclasses of tuples (and its subtypes) be compared either. "Magic" I did previously in objtype.c covers only one argument (lhs is many), so we're in trouble when lhs is native type - there's no other option besides handling rhs in special manner. Fortunately, this patch outlines approach with fast path for native types.