Introduce refactored wiki parser and renderer

This is a half-way through a big refactoring of the parsing and
rendering infrastructure. The main change is to separate the parse and
render trees, which makes the code a lot cleaner. The new parser isn't
yet functional enough to replace the existing parser so for the moment
you have to manually invoke it with `$tw.testNewParser()` in your
browser console. I really ought to use branches for this kind of
thing...

core/modules/widgets/view/viewers/link.js

@@ -0,0 +1,44 @@
+/*\
+title: $:/core/modules/widgets/view/viewers/link.js
+type: application/javascript
+module-type: newfieldviewer
+
+A viewer for viewing tiddler fields as a link
+
+\*/
+(function(){
+
+/*jslint node: true, browser: true */
+/*global $tw: false */
+"use strict";
+
+var LinkViewer = function(viewWidget,tiddler,field,value) {
+	this.viewWidget = viewWidget;
+	this.tiddler = tiddler;
+	this.field = field;
+	this.value = value;
+};
+
+LinkViewer.prototype.render = function() {
+	var parseTree = [];
+	if(this.value === undefined) {
+		parseTree.push({type: "text", text: ""});
+	} else {
+		parseTree.push({
+			type: "widget",
+			tag: "link",
+			attributes: {
+				to: {type: "string", value: this.value}
+			},
+			children: [{
+				type: "text",
+				text: this.value
+			}]
+		})
+	}
+	return this.viewWidget.renderer.renderTree.createRenderers(this.viewWidget.renderer.renderContext,parseTree);
+};
+
+exports.link = LinkViewer;
+
+})();