add initial MerkleTree implementation (#13)

Add initial MerkleTree implementation, which is a wrapper on top of
Plonky2's MerkleTree, with the idea that the future iteration will
replace it by the MerkleTree specified at
https://0xparc.github.io/pod2/merkletree.html .

README.md

@@ -2,4 +2,5 @@
 
 Specification can be found at the [`book`](https://github.com/0xPARC/pod2/tree/main/book) directory, a live rendered version at: https://0xparc.github.io/pod2/
 
-Requires Rust nightly: `rustup override set nightly`.
+## Usage
+- Run tests: `cargo test --release`