Returning tuples is often confusing as they don't convey meaning and it
should be used only when there is no possible ambiguity.
For `MerkleStore`, we had a couple of tuples being returned, and reading
the implementation was required in order to distinguish if they were
leaf values or computed roots.
This commit introduces two containers that will self-document these
returns: `RootPath` and `ValuePath`. It will also update `set_node` to
return both the new root & the new path, so we can prevent duplicated
traversals downstream when updating a node (one to update, the second to
fetch the new path/root).
A Merkle path is a vector of nodes, regardless of the Merkle tree
implementation.
This commit introduces an encapsulation for such vector, also to provide
functionality that is common between different algorithms such as
opening verification.
related issue: #36
