Fractions with min and max previously mentioned.

        * numbers.c (scm_max, scm_min): For inum/frac, frac/inum, big/frac,
        frac/big and frac/frac, use scm_less_p for exact comparison.

        * tests/numbers.test (max, min): Exercise some inum/frac, frac/inum,
        big/frac, frac/big and frac/frac cases.