perf(bn254): optimize Miller loop

[yelhousni]

Description

In non-native arithmetic subtractions are costlier than additions. In this PR we re-arrange some subs into adds.

Type of change

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

How has this been tested?

All pairing tests pass.

How has this been benchmarked?

This PR saves 22,462 scs in the ECPAIR precompile of size 4. The bigger the size the more it saves (given that new points are added to the multi-Miller loop).

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]

LGTM! One small comment about refactor only.

[ivokub]

Perfect! Thanks for addressing the comments.