This yields a pretty significant performance boost to pidigits and there are still some improvements to be made. Here are my numbers:

amd64 w/ bignum:
pidigits 10000
        gcc -O2 pidigits.c -lgmp        2.10u 0.00s 2.10r
        gc pidigits     22.92u 0.02s 22.97r
        gc_B pidigits   22.62u 0.00s 22.65r

amd64 w/ big:
pidigits 10000
        gcc -O2 pidigits.c -lgmp        2.09u 0.02s 2.11r
        gc pidigits     12.68u 0.04s 12.72r
        gc_B pidigits   12.71u 0.03s 12.75r

386 w/ bignum:
pidigits 10000
        gcc -O2 pidigits.c -lgmp        2.09u 0.00s 2.09r
        gc pidigits     44.30u 0.01s 44.35r
        gc_B pidigits   44.29u 0.03s 44.35r

386 w/ big:
pidigits 10000
        gcc -O2 pidigits.c -lgmp        2.10u 0.00s 2.10r
        gc pidigits     22.70u 0.06s 22.79r
        gc_B pidigits   22.80u 0.09s 22.91r

R=rsc, gri
CC=golang-dev
https://golang.org/cl/881050