Init merkle-mountain-range

The MMR code is copy from https://github.com/nervosnetwork/ckb/pull/1598

src/lib.rs

@@ -0,0 +1,14 @@
+mod error;
+mod helper;
+mod merge;
+mod mmr;
+mod mmr_store;
+#[cfg(test)]
+mod tests;
+pub mod util;
+
+pub use error::{Error, Result};
+pub use helper::{leaf_index_to_mmr_size, leaf_index_to_pos};
+pub use merge::Merge;
+pub use mmr::{MerkleProof, MMR};
+pub use mmr_store::MMRStore;