feat: add TriePathIter for iterating over the nodes along a path

[demosdemon]

This is used to walk along a path to yield all of the edges. In proof verification, this is used to verify the paths of the proof are valid.

[rkuris]

Let's chat about this and how it differs from the existing MerkleNodeIter. The existing code is tested heavily as it forms the basis of MerkleKeyValueIter.

If this code is better/faster/more generic, lets describe which and the motivation for a new iterator more clearly in the PR description.

[demosdemon]

Let's chat about this and how it differs from the existing MerkleNodeIter. The existing code is tested heavily as it forms the basis of MerkleKeyValueIter.

MerkleNodeIter requires a TrieReader and yields nodes as SharedNodes. They have different enough functionality that it isn't easy to replace one with the other. But, it would be possible to make TriePathIter replace MerkleNodeIter with some adjustments. However, the inverse wouldn't be true due to the dependency on TrieReader and SharedNode.