Further improve performance of Document creation.

This speeds up the create test by about **25-30%** by avoiding the call
to `util.assign()` in the `createDocument()` function. Instead of that
`Document#document` is now an accessor on the prototype, `defaultView`
is an instance of a new `DefaultView` class, whose instances have links
to the `document`, and finally the interesting constructors that are
exposed via `Document` instances are all installed on the `Document` and
`DefaultView` prototypes instead of the instances.

Author: bmeurer

src/undom.js

@@ -151,9 +151,32 @@ class Element extends Node {
 }
 
 
+class Event {
+	constructor(type, opts) {
+		this.type = type;
+		this.bubbles = !!(opts && opts.bubbles);
+		this.cancelable = !!(opts && opts.cancelable);
+	}
+	stopPropagation() {
+		this._stop = true;
+	}
+	stopImmediatePropagation() {
+		this._end = this._stop = true;
+	}
+	preventDefault() {
+		this.defaultPrevented = true;
+	}
+}
+
+
 class Document extends Element {
 	constructor() {
 		super(9, '#document');			// DOCUMENT_NODE
+		this.defaultView = new DefaultView(this);
+	}
+
+	get document() {
+		return this;
 	}
 
 	createElement(type) {
@@ -166,37 +189,28 @@ class Document extends Element {
 		return element;
 	}
 
-
 	createTextNode(text) {
 		return new Text(text);
 	}
 }
 
+assign(Document.prototype, { Document, Node, Text, Element, SVGElement: Element, Event });
 
-class Event {
-	constructor(type, opts) {
-		this.type = type;
-		this.bubbles = !!(opts && opts.bubbles);
-		this.cancelable = !!(opts && opts.cancelable);
-	}
-	stopPropagation() {
-		this._stop = true;
-	}
-	stopImmediatePropagation() {
-		this._end = this._stop = true;
-	}
-	preventDefault() {
-		this.defaultPrevented = true;
+
+class DefaultView {
+	constructor(document) {
+		this.document = document;
 	}
 }
 
+assign(DefaultView.prototype, { Document, Node, Text, Element, SVGElement: Element, Event });
+
 
 /** Create a minimally viable DOM Document
  *	@returns {Document} document
  */
 export default function createDocument() {
 	let document = new Document();
-	assign(document, document.defaultView = { document, Document, Node, Text, Element, SVGElement: Element, Event });
 	document.appendChild(
 		document.documentElement = document.createElement('html')
 	);