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/super/super_in_inherited_method.wren

@@ -6,4 +6,4 @@ class A {
 
 class B is A {}
 
-IO.print((new B).callSuperToString) // expect: instance of B
+IO.print(B.new().callSuperToString) // expect: instance of B