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

test/node_smoke_tests/lib/ensure_iterability_es6.js

@@ -0,0 +1,25 @@
+/* jshint esnext: true */
+
+"use strict";
+
+var assert = require( "assert" );
+
+module.exports = function ensureIterability() {
+	require( "jsdom" ).env( "", function( errors, window ) {
+		assert.ifError( errors );
+
+		var i,
+			ensureJQuery = require( "./ensure_jquery" ),
+			jQuery = require( "../../../dist/jquery.js" )( window ),
+			elem = jQuery( "<div></div><span></span><a></a>" ),
+			result = "";
+
+		ensureJQuery( jQuery );
+
+		for ( i of elem ) {
+			result += i.nodeName;
+		}
+
+		assert.strictEqual( result, "DIVSPANA", "for-of doesn't work on jQuery objects" );
+	} );
+};