Make constructors just methods.

* Eliminate "new" reserved word.
* Allow "this" before a method definition to define a constructor.
* Only create a default constructor for classes that don't define one.

test/language/this/closure.wren

@@ -1,7 +1,7 @@
 class Foo {
-  getClosure { new Fn { toString } }
+  getClosure { Fn.new { toString } }
   toString { "Foo" }
 }
 
-var closure = (new Foo).getClosure
+var closure = Foo.new().getClosure
 IO.print(closure.call()) // expect: Foo