This rewrites the class allocator search to be faster. Instead of using
RCLASS_SUPER, which is now even slower due to Box, we can scan the
superclasses list to find a class where the allocator is defined.
This also disallows allocating from an ICLASS. Previously I believe that
was only done for FrozenCore, and that was changed in
e596cf6e93dbf121e197cccfec8a69902e00eda3.
32e6dc0f31