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/limit/long_function.wren

@@ -1,4 +1,4 @@
-new Fn {
+Fn.new {
   IO.print(1) // expect: 1
   IO.print(2) // expect: 2
   IO.print(3) // expect: 3