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/io/print_bad_to_string.wren

@@ -2,4 +2,4 @@ class BadToString {
   toString { 3 }
 }
 
-IO.print(new BadToString) // expect: [invalid toString]
+IO.print(BadToString.new()) // expect: [invalid toString]