feat!: MerkleTree-specific Type Aliases (#251)

This PR introduces type aliases:
 - `MerkleTreeNodeIndex` for `usize`
 - `MerkleTreeLeafIndex` for `usize`
 - `MerkleTreeHeight` for `u32`
with the objective of documenting the conventions for
arithmetic with these index types.

Also:
 - Public function `nodes()` dropped in favor of `node(i)`.
 - Public function `leafs()` now returns an iterator instead
   of a slice.
 - Private function `authentication_structure_node_indices` is
   marked `pub` as it is needed downstream.

Displaces #250.

---------

Co-authored-by: Jan Ferdinand Sauer <ferdinand@neptune.cash>

Author: aszepieniec

twenty-first/benches/merkle_tree_authenticate.rs

@@ -56,8 +56,8 @@ impl Default for MerkleTreeSampler {
 }
 
 impl MerkleTreeSampler {
-    fn num_leafs(&self) -> usize {
-        1 << self.tree_height
+    fn num_leafs(&self) -> MerkleTreeLeafIndex {
+        (1 << self.tree_height) as MerkleTreeLeafIndex
     }
 
     fn leaf_digests(&mut self) -> Vec<Digest> {
@@ -72,7 +72,7 @@ impl MerkleTreeSampler {
         MerkleTree::par_new(&leaf_digests).unwrap()
     }
 
-    fn indices_to_open(&mut self) -> Vec<usize> {
+    fn indices_to_open(&mut self) -> Vec<MerkleTreeLeafIndex> {
         (0..self.num_opened_indices)
             .map(|_| self.rng.random_range(0..self.num_leafs()))
             .collect()