chore($parse): convert parser() and lex() to prototype-based code

This reduces memory consumption of parsed angular expressions and
speeds up parsing.

This JSPerf case demonstrates the performance boost:
http://jsperf.com/closure-vs-prototype-ngparser

Chrome: 1.5–2x boost
FF: slightly slower (I would love to know why)
IE: 4x boost

To be clear, this doesn't have any impact on runtime performance
of expressions as demostrated in this JSPerf:
http://jsperf.com/angular-parser-changes

Closes #3681

test/ng/parseSpec.js

@@ -2,6 +2,15 @@
 
 describe('parser', function() {
   describe('lexer', function() {
+    var lex;
+
+    beforeEach(function () {
+      lex = function () {
+        var lexer = new Lexer();
+        return lexer.lex.apply(lexer, arguments);
+      };
+    });
+
     it('should tokenize a string', function() {
       var tokens = lex("a.bc[22]+1.3|f:'a\\\'c':\"d\\\"e\"");
       var i = 0;