feat(bw6-761): Miller loop using direct sextic extension

[yelhousni]

Description

This corresponds to the gnark circuit and is needed to compute the same residueWitness in- and out-circuit for Consensys/gnark#1365.

Type of change

Please delete options that are not relevant.

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

How has this been tested?

Please describe the tests that you ran or implemented to verify your changes. Provide instructions so we can reproduce.

Added tests for the new E6D struct in internal/fptower/e6_direct_test.go and pairing computation using towered and direct extensions in pairing_test.go.

How has this been benchmarked?

MillerLoopDirect is much slower than MillerLoop but it is only used in the hint computation for the gnark circuit.

benchmark                 tower ns/op   direct ns/op     delta
BenchmarkMillerLoop-8     1195105       3922503       +228.21%

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]

Small cosmetic comments, otherwise looks perfect. Didn't check fully the direct mulMontgomery equations, but as the tests check compatibility with fp2fp3 tower multiplication, then should be good.

Thanks!