No more default constructors.

Fixes #296.

test/core/sequence/is_empty.wren

@@ -2,6 +2,7 @@ IO.print([].isEmpty) // expect: true
 IO.print([1].isEmpty) // expect: false
 
 class InfiniteSequence is Sequence {
+  construct new() {}
   iterate(iterator) { true }
   iteratorValue(iterator) { iterator }
 }