Ruby-GMP is significantly faster than Ruby BigNum,
and not much slower than direct C GMP,
but only on big numbers.

BigNum performs better on small numbers, but that's
mostly due to overhead of Ruby-GMP, not libGMP.
You can test where switch is reasonable using benchmarks 3 and 4.

Provided benchmarks check:
    benchmark 1		multiplication of big numbers
    benchmark 2		addition of big numbers
    benchmark 3		addition on various sizes
    benchmark 4		multiplication on various sizes
