math/g-bin.c, math/g-prime.c: Fix type incompatibility.

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.