feat: wrap existing hash-to-field for implementing hash.Hash interface

[ivokub]

Description

We already provide default hash-to-field in fp and fr packages. However, right now it is only a method, but it would be beneficial if we would be able to implement hash.Hash (from Go crypto/hash) so that we can have custom hash-to-field functions in gnark. This PR adds the wrapper such that the output is equal to naively calling fr.Hash on the same byte slice input. Also add a basic test.

Related to Consensys/gnark#847 as we would need to be able to swap hash-to-field in Plonk prover and verifier.

Type of change

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

How has this been tested?

• test to ensure that interface output is equivalent to the existing fr.Hash output.

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