feat: field.Hash

[Tabaie]

For FS commitments it is needed to hash to fr, so I'm moving the hashToFp code from the HashToCurve modules into every Fp package.
One concern is that the hash function uses some ExpandMsgXmd function, elsewhere in the gnark-crypto codebase. So as it stands field packages can no longer by copy-pasted into other projects without importing gnark-crypto.

[gbotrel]

looks good to me 👍
Small change: package hash_utils should be hashutils (using Google style guide as reference, but I know some linters complain all the same--> https://google.github.io/styleguide/go/decisions#package-names )

We can keep this package in internal/ for now, but in future it may be merged with other common pieces between field element pacakges 👍