fix(ecdsa): correct short weierstrass curve eqn

[Maddiaa0]

Description

Bumped into an issue creating circuits proving secp256r1 circuits, this pr pushes a fix checking the pk is on the curve, and also corrects curve eqn for curves with an a value.

Checklist:

• I have reviewed my diff in github, line by line.
• Every change is related to the PR description.
• The branch has been merged with/rebased against the head of its merge target.
• There are no unexpected formatting changes, superfluous debug logs, or commented-out code.
• There are no circuit changes, OR a cryptographer has been assigned for review.
• New functions, classes, etc. have been documented according to the doxygen comment format. Classes and structs must have @brief describing the intended functionality.
• If existing code has been modified, such documentation has been added or updated.
• No superfluous include directives have been added.
• I have linked to any issue(s) it resolves.
• I'm happy for the PR to be merged at the reviewer's next convenience.

[codygunton]

Looks good, thanks!