feat: add merkle path containers and return them on tree update

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).

src/merkle/path.rs

@@ -82,3 +82,27 @@ impl IntoIterator for MerklePath {
         self.nodes.into_iter()
     }
 }
+
+// MERKLE PATH CONTAINERS
+// ================================================================================================
+
+/// A container for a [Word] value and its [MerklePath] opening.
+#[derive(Clone, Debug, Default, PartialEq, Eq)]
+pub struct ValuePath {
+    /// The node value opening for `path`.
+    pub value: Word,
+    /// The path from `value` to `root` (exclusive).
+    pub path: MerklePath,
+}
+
+/// A container for a [MerklePath] and its [Word] root.
+///
+/// This structure does not provide any guarantees regarding the correctness of the path to the
+/// root. For more information, check [MerklePath::verify].
+#[derive(Clone, Debug, Default, PartialEq, Eq)]
+pub struct RootPath {
+    /// The node value opening for `path`.
+    pub root: Word,
+    /// The path from `value` to `root` (exclusive).
+    pub path: MerklePath,
+}