perf(sw_emulated): optimize jointScalarMulGeneric

[yelhousni]

Description

jointScalarMulGeneric is used to compute [u]P+[v]Q when the GLV endomorphism is not a available. It was implementing it naively as most of the curves of interest to us have GLV. But users might want for example to verify ECDSA with NIST curves e.g. P-256, P-384 — in which case this PR might come in handy.

Type of change

• New feature (non-breaking change which adds functionality)

How has this been tested?

Added test TestJointScalarMul4 and TestJointScalarMulEdgeCases4 (edge casess involving 0-scalars and (0,0)-points) for P-256 curve (no GLV).

How has this been benchmarked?

old: 1418482 scs
new: 1186170 scs

Checklist:

• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• I did not modify files generated from templates
• golangci-lint does not output errors locally
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules

[ivokub]

Perfect!