perf: BW6 pairing computation using non-native Eval

[ivokub]

Description

This PR refactors the methods in the fields_bw6761 package to use the Field.Eval methods introduced in #1299. Depends on it being merged first.

Currently we use simple schoolbook multiplication formulas, but can have better performance with specialized formulas a la Toom-Cook etc. Currently didn't find a good way to give a multivariate representation to them.

Type of change

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

How has this been tested?

• TestSquareVariantsFp6
• TestMulVariantsFp6
• TestFp6Mul023By023Variants
• TestFp6MulBy02345Variants
• TestFp6CyclotomicSquareKarabina12345Variants

How has this been benchmarked?

• Single Miller loop before: 6500708, now: 3841000
• Single Miller loop fixed G2 before: 5344302, now: 2680076
• FinalExp before: 5245872, now: 3362746
• Full pairing before: 11486969, now: 6947630
• Full pairing fixed G2 before: 10440385, now: 5888826

But keep in mind that I only measured single operations so they include range checks for the initial witness.

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