Init merkle-mountain-range

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

src/error.rs

@@ -0,0 +1,12 @@
+pub use failure::Fail;
+pub type Result<T> = ::std::result::Result<T, Error>;
+
+#[derive(Fail, Debug, PartialEq, Eq, Clone)]
+pub enum Error {
+    #[fail(display = "Get root on an empty MMR")]
+    GetRootOnEmpty,
+    #[fail(display = "Inconsistent store")]
+    InconsistentStore,
+    #[fail(display = "Store error {}", _0)]
+    StoreError(String),
+}