math/big: test and optimize Exp(2, y, n) for large y, odd n

math/big: test and optimize Exp(2, y, n) for large y, odd n

The Montgomery multiply code is applicable to this case
but was being bypassed. Don't do that.

The old test len(x) > 1 was really just a bad approximation to x > 1.

name    old time/op  new time/op  delta
Exp-8   5.56ms ± 4%  5.73ms ± 3%     ~     (p=0.095 n=5+5)
Exp2-8  7.59ms ± 1%  5.66ms ± 1%  -25.40%  (p=0.008 n=5+5)

This comes up especially when doing Fermat (Miller-Rabin)
primality tests with base 2.

Change-Id: I4cc02978db6dfa93f7f3c8f32718e25eedb4f5ed
Reviewed-on: https://go-review.googlesource.com/30708
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>