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.

example/import-module/lovecraft.wren

@@ -1,7 +1,7 @@
 import "cthulu" for Cthulu
 
 class Lovecraft {
-  say { (new Cthulu).message }
+  say() { Cthulu.new().message }
 }
 
-IO.print((new Lovecraft).say)
+IO.print(Lovecraft.new().say())