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;

core/modules/filters/sameday.js

@@ -20,12 +20,9 @@ exports.sameday = function(source,operator,options) {
 		fieldName = operator.suffix || "modified",
 		targetDate = (new Date($tw.utils.parseDate(operator.operand))).setHours(0,0,0,0);
 	// Function to convert a date/time to a date integer
-	var isSameDay = function(dateField) {
-			return (new Date(dateField)).setHours(0,0,0,0) === targetDate;
-		};
 	source(function(tiddler,title) {
-		if(tiddler && tiddler.fields[fieldName]) {
-			if(isSameDay($tw.utils.parseDate(tiddler.fields[fieldName]))) {
+		if(tiddler) {
+			if(tiddler.getFieldDay(fieldName) === targetDate) {
 				results.push(title);
 			}
 		}

core/modules/tiddler.js

@@ -108,4 +108,17 @@ exports.isEqual = function(tiddler,excludeFields) {
 	return differences.length === 0;
 };
 
+exports.getFieldDay = function(field) {
+	if(this.cache && this.cache.day && $tw.utils.hop(this.cache.day,field) ) {
+		return this.cache.day[field];
+	}
+	var day = "";
+	if(this.fields[field]) {
+		day = (new Date($tw.utils.parseDate(this.fields[field]))).setHours(0,0,0,0);
+	}
+	this.cache.day = this.cache.day || {};
+	this.cache.day[field] = day;
+	return day;
+};
+
 })();