Use more conventional syntax for constructors.

They are now invoked like "new Foo".
Also, superclass constructors are now much less semantically
and syntactically weird. Since the instance is created before
any constructor is called, there's no point in time where the
instance isn't there.

test/field/use_before_set.wren

@@ -3,7 +3,7 @@ class Foo {
   init { _field = "value" }  // ...before an assignment to it.
 }
 
-var foo = Foo.new
+var foo = new Foo
 // But invoke them in the right order.
 foo.init
 foo.write // expect: value