Freeze the entire tiddler object

As seen in the first pass of #2247, it was previously inadvertently
possible for callers to modify the tiddler object itself by adding and
replacing properties.

boot/boot.js

@@ -812,6 +812,7 @@ $tw.Tiddler = function(/* [fields,] fields */) {
 	}
 	// Freeze the tiddler against modification
 	Object.freeze(this.fields);
+	Object.freeze(this);
 };
 
 $tw.Tiddler.prototype.hasField = function(field) {