efficient bulk insert

(cherry picked from commit c83a4a4dc0)

src/merkleTree.js

@@ -22,13 +22,13 @@ class MerkleTree {
    * @param {hashFunction} [options.hashFunction] Function used to hash 2 leaves
    * @param [options.zeroElement] Value for non-existent leaves
    */
-  constructor(levels, elements = [], { hashFunction = defaultHash, zeroElement = DEFAULT_ZERO } = {}) {
+  constructor(levels, elements = [], { hashFunction, zeroElement = DEFAULT_ZERO } = {}) {
     this.levels = levels
     this.capacity = 2 ** levels
     if (elements.length > this.capacity) {
       throw new Error('Tree is full')
     }
-    this._hash = hashFunction
+    this._hash = hashFunction || defaultHash
     this.zeroElement = zeroElement
     this._zeros = []
     this._zeros[0] = zeroElement