Chores: export the unrolled FFT64 out of SIS

[AlexandreBelling]

Description

This PR exports the functions Fft64 and ComputeTwiddle

Type of change

No procedural changes in the code

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

[github-actions]

📦 github.com/consensys/gnark-crypto/ecc/bls12-381
❌ TestPairing 0s

📦 github.com/consensys/gnark-crypto/ecc/bn254/fr/gkr
❌ TestMimc 0s

    gkr_test.go:177: Entered test orchestrator, assigning and randomizing inputs
    gkr_test.go:190: Selected inputs for test
    gkr_test.go:343: Evaluating all circuit wires
    gkr_test.go:345: Circuit evaluation complete