math/big: add 4-bit, fixed window exponentiation.
A 4-bit window is convenient because 4 divides both 32 and 64,
therefore we never have a window spanning words of the exponent.
Additionaly, the benefit of a 5-bit window is only 2.6% at 1024-bits
and 3.3% at 2048-bits.
This code is still not constant time, however.
benchmark old ns/op new ns/op delta
BenchmarkRSA2048Decrypt
17108590 11180370 -34.65%
Benchmark3PrimeRSA2048Decrypt
13003720 7680390 -40.94%
R=gri
CC=golang-dev
https://golang.org/cl/
6716048