New implementation of incremental merkle tree

This is a new implementation of the incremental merkle tree used by our
scheme to witness commitments to spendable value. It serves as a fixed-sized
accumulator.

This new construction has a much simpler API surface area, avoids memory
safety issues, remains pruned at all times, avoids serialization edge cases,
has more efficient insertion, and is abstract over the depth and hash
function used at the type level.

Further, it lays the groundwork for efficient "fast-forwarding" of witnesses
into the tree as the treestate is updated.

src/coins.h

@@ -17,8 +17,7 @@
 #include <boost/foreach.hpp>
 #include <boost/unordered_map.hpp>
 #include "zerocash/IncrementalMerkleTree.h"
-
-static const unsigned int INCREMENTAL_MERKLE_TREE_DEPTH = 20;
+#include "zcash/IncrementalMerkleTree.hpp"
 
 /** 
  * Pruned version of CTransaction: only retains metadata and unspent transaction outputs