A concrete proposal for handling infs with mpz_cmp_d, as previously
mentioned.

        * numbers.c (xmpz_cmp_d): New macro, handling infs if gmp doesn't.
        (scm_num_eq_p, scm_less_p, scm_max, scm_min): Use it.

        * tests/numbers.test (=, <, max, min): Add tests of bignum/inf
        combinations.