group: Avoid using infinity field directly in other modules

[real-or-random]

Minor refactoring to make the abstraction cleaner

[hebasto]

Concept ACK.

Some other places still remain:

$ git grep -e "\.infinity" -- src ':(exclude)src/group_impl.h'
src/ecmult_const_impl.h:    p.infinity = 0;
src/tests.c:    SECP256K1_CHECKMEM_CHECK(&ge.infinity, sizeof(ge.infinity));
src/tests_exhaustive.c:            zless_gej.infinity = groupj[j].infinity;
src/tests_exhaustive.c:                CHECK(group[i].infinity == 0);
src/tests_exhaustive.c:                CHECK(generated.infinity == 0);

[theStack]

Concept ACK