build: add tentative tree API

crates/committer/src/lib.rs

@@ -1,3 +1,5 @@
+pub mod patricia_merkle_tree;
+
 // TODO(Dori, 3/3/2024): Delete this dummy code.
 pub fn dummy() -> u8 {
     7

crates/committer/src/patricia_merkle_tree.rs

@@ -0,0 +1,2 @@
+pub mod node;
+pub mod tree;

crates/committer/src/patricia_merkle_tree/node.rs

@@ -0,0 +1 @@
+pub struct Node {}

crates/committer/src/patricia_merkle_tree/tree.rs

@@ -0,0 +1,12 @@
+use crate::patricia_merkle_tree::node::Node;
+
+trait Tree {
+    /// Returns the node with given full (Merkle) index, if it exists.
+    fn get_node(full_index: u128) -> Option<Node>;
+
+    /// Returns the root if the tree is not empty.
+    fn get_root() -> Option<Node>;
+
+    /// Computes the hash of the given node.
+    fn compute_hash(node: Node) -> [u8; 32];
+}