math/big: implement Atkin's ModSqrt for 5 mod 8 primes

For primes congruent to 5 mod 8 there is a simple deterministic
method for calculating the modular square root due to Atkin,
using one exponentiation and 4 multiplications.

A. Atkin.  Probabilistic primality testing, summary by F. Morain.
Research Report 1779, INRIA, pages 159–163, 1992.

This increases the speed of modular square roots for these primes
considerably.

name                old time/op  new time/op  delta
ModSqrt231_5Mod8-4  1.03ms ± 2%  0.36ms ± 5%  -65.06%  (p=0.008 n=5+5)

Change-Id: I024f6e514bbca8d634218983117db2afffe615fe
Reviewed-on: https://go-review.googlesource.com/99615


Reviewed-by: Robert Griesemer <gri@golang.org>