crypto/elliptic: generalize elliptic.Curve to allow A != -3

[collinc97]

Fixes #26776

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[collinc97]

I signed it!

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[cag]

Confirm I'm ok with my commits being contributed to this project!

[Hirama]

When merge?

[shazow]

Do we need to override the cla:no label here somehow to get it reviewed?

[mvdan]

Looks like the proposal issue was declined. Leaving it up to @FiloSottile to decide what to do here, but I assume we can just close this PR.

[eliwjones]

I went ahead and pulled the code from the PR out and put a usable version here (tests and example included):

https://github.com/eliwjones/crypto

The main secp256k1 stuff I see out and about are wrappers of C code.

I have not seen a satisfactorily trustworthy native Go implementation.. so, if anyone knows of one, please post it here.

Linking to issue thread as well:
#26776 (comment)

[odeke-em]

The subject proposal/issue was closed in #26776 (comment), thus closing this PR too. We appreciate your contributions to the Go project @collinc97, and look forward to interacting with you even more in the future. Thank you.