crypto/rsa: precompute moduli

crypto/rsa: precompute moduli

This change adds some private fields to PrecomputedValues.

If applications were for some reason manually computing the
PrecomputedValues, which they can't do anymore, things will still work
but revert back to the unoptimized path.

name                    old time/op  new time/op  delta
DecryptPKCS1v15/2048-8  1.40ms ± 0%  1.24ms ± 0%  -10.98%  (p=0.000 n=10+8)
DecryptPKCS1v15/3072-8  4.14ms ± 0%  3.78ms ± 1%   -8.55%  (p=0.000 n=10+10)
DecryptPKCS1v15/4096-8  9.09ms ± 0%  8.62ms ± 0%   -5.20%  (p=0.000 n=9+8)
EncryptPKCS1v15/2048-8   139µs ± 0%   138µs ± 0%     ~     (p=0.436 n=9+9)
DecryptOAEP/2048-8      1.40ms ± 0%  1.25ms ± 0%  -11.01%  (p=0.000 n=9+9)
EncryptOAEP/2048-8       139µs ± 0%   139µs ± 0%     ~     (p=0.315 n=10+10)
SignPKCS1v15/2048-8     1.53ms ± 0%  1.29ms ± 0%  -15.93%  (p=0.000 n=9+10)
VerifyPKCS1v15/2048-8    138µs ± 0%   138µs ± 0%     ~     (p=0.052 n=10+10)
SignPSS/2048-8          1.54ms ± 0%  1.29ms ± 0%  -15.89%  (p=0.000 n=9+9)
VerifyPSS/2048-8         139µs ± 0%   139µs ± 0%     ~     (p=0.442 n=8+8)

Change-Id: I843c468db96aa75b18ddff17cec3eadfb579cd0e
Reviewed-on: https://go-review.googlesource.com/c/go/+/445020
Reviewed-by: Joedian Reid <joedian@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Roland Shoemaker <roland@golang.org>