mtx: expose commit method returning a tx and view.

lib/primitives/mtx.js

@@ -1401,6 +1401,15 @@ MTX.prototype.toTX = function toTX() {
   return new TX().inject(this);
 };
 
+/**
+ * Convert the MTX to a TX.
+ * @returns {Array} [tx, view]
+ */
+
+MTX.prototype.commit = function commit() {
+  return [this.toTX(), this.view];
+};
+
 /**
  * Instantiate MTX from TX.
  * @param {TX} tx