Core: Make jQuery objects iterable

Make iterating over jQuery objects possible using ES 2015 for-of:

    for ( node of $( "<div id=narwhal>" ) ) {
        console.log( node.id ); // "narwhal"
    }

Fixes gh-1693

src/core.js

@@ -425,6 +425,16 @@ jQuery.extend({
 	support: support
 });
 
+// JSHint would error on this code due to the Symbol not being defined in ES5.
+// Defining this global in .jshintrc would create a danger of using the global
+// unguarded in another place, it seems safer to just disable JSHint for these
+// three lines.
+/* jshint ignore: start */
+if ( typeof Symbol === "function" ) {
+	jQuery.fn[ Symbol.iterator ] = arr[ Symbol.iterator ];
+}
+/* jshint ignore: end */
+
 // Populate the class2type map
 jQuery.each("Boolean Number String Function Array Date RegExp Object Error".split(" "),
 function(i, name) {