refactor!(MerkleTree): Use index type aliases

This commit introduces type aliases:
 - `MerkleTreeNodeIndex` for `u64`
 - `MerkleTreeLeafIndex` for `u64`
 - `MerkleTreeHeight` for `u32`.

It also refactors `MerkleTree` to avoid using `usize` where-ever
possible, particularly in public functions. Instead, these types
aliases are used.

`MerkleTree` internally stores a `Vec<Digest>`, which induces two
limitations:
 1. The index type is `usize`, which complicates matters when
    targeting compilation on 32-bit machines (including 32-bit WASM).
 2. This vector can store at most 2^25 nodes, whereas we would like
    functions associated with Merkle trees to work for Merkle trees
    that are far larger (without needing to store all the internal
    nodes explicitly).

The second limitation persists. However, the first limitation is fixed
by this commit.

Also, public functions `nodes()` and `leafs()` now return iterators
instead of slices. As these functions were public, this change is
breaking.

Addresses #250.

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()