math/big: improved karatsuba calibration code, better mul benchmark
An attempt to profit from CL
6176043 (fix to superpolinomial
runtime of karatsuba multiplication) and determine a better
karatsuba threshold. The result indicates that 32 is still
a reasonable value. Left the threshold as is (== 32), but
made some minor changes to the calibrate code which are
worthwhile saving (use of existing benchmarking code for
better results, better use of package time).
R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/
6260062