math/big: fix superpolynomial complexity in Karatsuba algorithm.
benchmark old ns/op new ns/op delta
BenchmarkExp3Power0x10 732 734 +0.27%
BenchmarkExp3Power0x40 834 836 +0.24%
BenchmarkExp3Power0x100 1600 1579 -1.31%
BenchmarkExp3Power0x400 3478 3417 -1.75%
BenchmarkExp3Power0x1000 19388 19229 -0.82%
BenchmarkExp3Power0x4000 160274 156881 -2.12%
BenchmarkExp3Power0x10000
1552050 1372058 -11.60%
BenchmarkExp3Power0x40000
27328710 15216920 -44.32%
BenchmarkExp3Power0x100000
612349000 131407100 -78.54%
BenchmarkExp3Power0x400000
44073524000 1122195000 -97.45%
R=golang-dev, mtj, gri, rsc
CC=golang-dev, remy
https://golang.org/cl/
6176043