math/big: use recursive subdivision for significant speedup

math/big: use recursive subdivision for significant speedup

This change adds the second aspect to the conversion code, the
use of large divisiors (powers of big base) to greatly speed up
the divsion of large numbers. Speedups of 30x are common in the
large cases. Also includes new tests and tuning code for the
key internal parameters.

R=gri
CC=golang-dev
https://golang.org/cl/5438058