Digest-generic MerkleTreeGadget

[tessico]

Description

Making the circuit equivalent of MerkleTree generic over a digest algorithm is more tricky, because both the MerkleTreeGadget and DigestAlgorithmGadget need to be implemented for PlonkCircuit.

I'm proposing two designs here. One is to add it as a generic parameter to MerkleTreeGadget, see the last commit. The other option is to add DigestAlgorithmGadget as an associated type on the MerkleTreeGadget trait, see here. Both options work, it's just a matter of convenience and ergonomics. Please let me know your opinions.

(Note that initially I thought that the associated type would not work in the case where in the future we need to provide multiple implementations of the MerkleTreeGadget trait. But since the trait already is generic over MerkleTreeScheme, which, implicitly, will be using the counterpart of DigestAlgorithmGadget, it should still be possible to provide multiple impl even with DigestAlgorithmGadget being an associated type. See the rust book for more explanation on the difference between the two.)

closes: #142

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[mrain]

Generally LGTM.

[tessico]

As per the discussion on the first commit, I've gone back to the initial solution by reverting the second commit.

[mrain]

Excellent work! Will approve if comments are addressed.

[mrain]

LGTM