math/g-bin.c, math/g-prime.c: Fix type incompatibility.
Callers of the abstract group API expect to pass in a pointer-to-
structure. The binary and prime group implementations expected a
pointer-to-pointer, which looks different. Change the way these work,
so that the group element is a structure holding a pointer, rather than
just a bare pointer. This doesn't make any difference on targets with
sane ABIs, but it fixes a potentially nasty problem on weirder
platforms.
Add a macro explaining this change so that users of this unstable
interface can cope with both versions.
~mdw / catacomb / commit