Replace hashCode with checksum

Hash codes are not collision free, so what we
actually need is a checksum.

src/ol/structs/checksum.js

@@ -0,0 +1,22 @@
+goog.provide('ol.structs.IHasChecksum');
+
+
+/**
+ * @typedef {string}
+ */
+ol.structs.Checksum;
+
+
+
+/**
+ * @interface
+ */
+ol.structs.IHasChecksum = function() {
+};
+
+
+/**
+ * @return {string} The checksum.
+ */
+ol.structs.IHasChecksum.prototype.getChecksum = function() {
+};