Rename zcash_mmr to zcash_history.

This crate will contain all chain history logic.

zcash_history/examples/lib/shared.rs

@@ -1,4 +1,4 @@
-use zcash_mmr:: {NodeData, Tree, Entry, EntryLink};
+use zcash_history:: {NodeData, Tree, Entry, EntryLink};
 
 pub struct NodeDataIterator {
     return_stack: Vec<NodeData>,

zcash_history/examples/long.rs

@@ -1,4 +1,4 @@
-use zcash_mmr::{Entry, EntryLink, NodeData, Tree};
+use zcash_history::{Entry, EntryLink, NodeData, Tree};
 
 #[path= "lib/shared.rs"]
 mod share;