crypto/elliptic: precompute ScalarBaseMult doublings

crypto/elliptic: precompute ScalarBaseMult doublings

name                    old time/op    new time/op    delta
pkg:crypto/ecdsa goos:darwin goarch:amd64
Sign/P224-16               250µs ± 2%      91µs ± 2%  -63.42%  (p=0.000 n=10+9)
Sign/P384-16               955µs ± 3%     311µs ± 2%  -67.48%  (p=0.000 n=10+10)
Sign/P521-16              2.74ms ± 2%    0.82ms ± 2%  -69.95%  (p=0.000 n=10+10)
Verify/P224-16             440µs ± 3%     282µs ± 5%  -35.94%  (p=0.000 n=9+10)
Verify/P384-16            1.72ms ± 2%    1.07ms ± 1%  -38.02%  (p=0.000 n=10+9)
Verify/P521-16            5.10ms ± 2%    3.18ms ± 3%  -37.70%  (p=0.000 n=10+10)
GenerateKey/P224-16        225µs ± 3%      67µs ± 4%  -70.42%  (p=0.000 n=9+10)
GenerateKey/P384-16        881µs ± 1%     241µs ± 2%  -72.67%  (p=0.000 n=10+10)
GenerateKey/P521-16       2.62ms ± 3%    0.69ms ± 3%  -73.78%  (p=0.000 n=10+9)
pkg:crypto/elliptic/internal/nistec goos:darwin goarch:amd64
ScalarMult/P224-16         219µs ± 4%     209µs ± 3%   -4.57%  (p=0.003 n=10+10)
ScalarMult/P384-16         838µs ± 2%     823µs ± 1%   -1.72%  (p=0.004 n=10+9)
ScalarMult/P521-16        2.48ms ± 2%    2.45ms ± 2%     ~     (p=0.052 n=10+10)
ScalarBaseMult/P224-16     214µs ± 4%      54µs ± 4%  -74.88%  (p=0.000 n=10+10)
ScalarBaseMult/P384-16     828µs ± 2%     196µs ± 3%  -76.38%  (p=0.000 n=10+10)
ScalarBaseMult/P521-16    2.50ms ± 3%    0.55ms ± 2%  -77.96%  (p=0.000 n=10+10)

Updates #52424
For #52182

Change-Id: I2be3c2b8cdeead512063ef489e43805f4ee71d0f
Reviewed-on: https://go-review.googlesource.com/c/go/+/404174
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Fernando Lobato Meeser <felobato@google.com>
Reviewed-by: Roland Shoemaker <roland@golang.org>