If category defined ivars or properties, these are added to the class
via a mulle_vararg builder sort of. So now we only need the offset, as
we have enough space in the instance. Where can we store the offset,
well a category is a methodlist, so why not there. How do we get the
methodlist. Change IMP from id, SEL, void * to id, method *, void *,
now suddenly we can get at an offset cheaply.
A kind of downside is that, each cache lookup now needs an additional memory read, to get the method and then the IMP.