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/function/newline_body.wren

@@ -1,4 +1,4 @@
-var f = new Fn {
+var f = Fn.new {
   // Hi.
 }
 IO.print(f.call()) // expect: null