Optimise sameday filter

I used this test:

console.time();for(var t=0; t<200; t++)
{$tw.wiki.filterTiddlers("[all[tiddlers+shadows]sameday[20170210]]");};c
onsole.timeEnd()

Before this patch, I got speeds of approx 190ms, versus 140ms
afterwards.

Note that the ability to add a cache property like this is only
possible because tiddler objects are immutable.

boot/boot.js

@@ -827,6 +827,7 @@ taking precedence to the right
 */
 $tw.Tiddler = function(/* [fields,] fields */) {
 	this.fields = Object.create(null);
+	this.cache = Object.create(null);
 	for(var c=0; c<arguments.length; c++) {
 		var arg = arguments[c],
 			src = (arg instanceof $tw.Tiddler) ? arg.fields : arg;