crypto/elliptic: use a 4-bit sliding window for P-521 ScalarMult

crypto/elliptic: use a 4-bit sliding window for P-521 ScalarMult

name                    old time/op    new time/op    delta
pkg:crypto/elliptic goos:darwin goarch:amd64
ScalarBaseMult/P521-16    1.63ms ± 4%    1.00ms ± 1%  -38.69%  (p=0.000 n=10+8)
ScalarMult/P521-16        1.65ms ± 4%    0.99ms ± 2%  -40.15%  (p=0.000 n=10+10)
pkg:crypto/ecdsa goos:darwin goarch:amd64
Sign/P521-16              1.67ms ± 1%    1.12ms ± 2%  -32.82%  (p=0.000 n=8+10)
Verify/P521-16            3.10ms ± 2%    2.00ms ± 2%  -35.54%  (p=0.000 n=9+10)
GenerateKey/P521-16       1.53ms ± 1%    0.98ms ± 2%  -35.81%  (p=0.000 n=9+10)

Change-Id: I109e821399d71330a77d105496e227746cc3ea0d
Reviewed-on: https://go-review.googlesource.com/c/go/+/320072
Trust: Filippo Valsorda <filippo@golang.org>
Run-TryBot: Filippo Valsorda <filippo@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Julie Qiu <julie@golang.org>