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/nonlocal/use_in_function.wren

@@ -1,5 +1,5 @@
 var Global = "global"
 
-new Fn {
+Fn.new {
   IO.print(Global) // expect: global
 }.call()