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