No more default constructors.

Fixes #296.

test/language/inheritance/is.wren

@@ -1,6 +1,12 @@
-class A {}
-class B is A {}
-class C is B {}
+class A {
+  construct new() {}
+}
+class B is A {
+  construct new() {}
+}
+class C is B {
+  construct new() {}
+}
 var a = A.new()
 var b = B.new()
 var c = C.new()