clarifying the merkletree package usage

[staheri14]

Problem

Inline with #109 and identified as part of this EPIC: celestiaorg/celestia-app#1296.

The HashNode and HashLeaf functions in the nmt package aim to satisfy the NodeHasher and LeafHasherz interfaces from the merkletree package.
Therefore, they need to comply with the signatures dictated by those interfaces. As a result, they cannot return any errors upon invalid inputs, as explained in issue #109. However, it is not clear whether this compliance is necessary, as pointed out by @evan-forbes. Therefore, this issue is to investigate the usage of the merkletree package in the nmt package and in celestia-app, celestia-core, and celestia-node. If no usage is found, the package should be removed from the set of dependencies.

[staheri14]

Sharing my findings here for future reference:

NMT repo: The usage of the merkletree package in the nmt repository is limited to a single dependency. Specifically, the nmt.Hasher type must implement the merkletree.TreeHasher interface to create Merkle range proofs using the merkletree package. This requirement is only applicable in the following lines:

nmt/proof_test.go

Lines 177 to 178 in eec4bc0

	subTreeHasher := newCachedSubtreeHasher(n.leafHashes, n.treeHasher)
	incompleteRange, err := merkletree.BuildRangeProof(start, end, subTreeHasher)

However, the NamespaceMerkleTree struct provides an alternative method for generating range proofs called buildRangeProof. Therefore, it is safe to remove the dependency on the merkletree package from the nmt repository.
Furthermore, the other repos i.e., celestia-app, celestia-core and celestia-node were also searched to determine if the Hasher type had ever been used as a merkletree.TreeHasher. However, no usage was found.

The necessary changes for this removal are available in the following pull request: #126.

[staheri14]

Based on the following, it seems to be fine to archive the merkletree package.
As of celestiaorg/celestia-app@0c7806a no usage was found for the merkletree package in the celestia-app repo.
As of celestiaorg/celestia-core@a11b8cb no usage was found for the merkletree package in the celestia-core repo.
As of celestiaorg/celestia-node@b7213bf no usage was found for the merkletree package in the celestia-node repo.

Awaiting celestia-node team's response on Slack to ensure archiving the merkletree repo is ok.
cc: @evan-forbes @rootulp

[staheri14]

Based on the response from the Celestia-node team, we are good to archive the merkletree repo.

[rootulp]

Cool! https://github.com/celestiaorg/merkletree was archived yesterday

[staheri14]

Cool! https://github.com/celestiaorg/merkletree was archived yesterday

@rootulp Great! so going to close this issue.