feat: allow hashing Fiat-Shamir transcript challenge without hashing to field first

[ivokub]

Description

Fixes #467.

This PR allows to avoid hashing to field before we hash the challenge name into the transcript. The problem with the current approach was that it wasn't that well compatible with gnark counterpart when doing recursion in-circuit.

The obvious solution was to add options to conditionally allow altering the behaviour of Fiat-Shamir instance, but this created another problem as currently the challenge names were already given as variadic arguments and we cannot have multiple.

So, I removed mandatory challenge name fixing when initializing transcript and added an option to optionally fix them. Now, if the challenge names are not given then we can bind to new challenges on the go.

This allowed to add the option for avoiding the hash to field before hashing the challenge name into the transcript. Added edge case check where if hash to field is requested but we do not have the method.

This fix breaks usage of Fiat-Shamir transcript, but it is fixed in gnark-crypto and I'm also pushing changes to gnark. See Consensys/gnark#974

Type of change

Please delete options that are not relevant.

• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

How has this been tested?

Tests work. Also checked that fixes the issue in zkevm.

How has this been benchmarked?

Not benchmarked.

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]

Seems there is some incompatibility with gnark still.

[github-actions]

📦 github.com/consensys/gnark-crypto/ecc/bn254/fr/mimc
❌ TestMiMCFiatShamir 0s

panic: runtime error: slice bounds out of range [:32] with capacity 31 [recovered]
	panic: runtime error: slice bounds out of range [:32] with capacity 31

goroutine 19 [running]:
testing.tRunner.func1.2({0x5ca380, 0xc0001c0168})
	/opt/hostedtoolcache/go/1.21.5/x64/src/testing/testing.go:1545 +0x238
testing.tRunner.func1()
	/opt/hostedtoolcache/go/1.21.5/x64/src/testing/testing.go:1548 +0x397
panic({0x5ca380?, 0xc0001c0168?})
	/opt/hostedtoolcache/go/1.21.5/x64/src/runtime/panic.go:914 +0x21f
github.com/consensys/gnark-crypto/ecc/bn254/fr/mimc.(*digest).Write(0xc0001b7cc0, {0xc0001de400, 0x1f, 0x1f})
	/home/runner/work/gnark-crypto/gnark-crypto/ecc/bn254/fr/mimc/mimc.go:106 +0x1b7
github.com/consensys/gnark-crypto/fiat-shamir.(*Transcript).ComputeChallenge(0xc000187710, {0x5de0cb, 0x2})
	/home/runner/work/gnark-crypto/gnark-crypto/fiat-shamir/transcript.go:132 +0x555
github.com/consensys/gnark-crypto/ecc/bn254/fr/mimc_test.TestMiMCFiatShamir(0x0?)
	/home/runner/work/gnark-crypto/gnark-crypto/ecc/bn254/fr/mimc/mimc_test.go:16 +0x45e
testing.tRunner(0xc0001976c0, 0x6209f8)
	/opt/hostedtoolcache/go/1.21.5/x64/src/testing/testing.go:1595 +0xff
created by testing.(*T).Run in goroutine 1
	/opt/hostedtoolcache/go/1.21.5/x64/src/testing/testing.go:1648 +0x3ad