No more default constructors.

Fixes #296.

test/language/this/nested_class.wren

@@ -1,4 +1,6 @@
 class Outer {
+  construct new() {}
+
   method {
     IO.print(this) // expect: Outer
 
@@ -6,6 +8,8 @@ class Outer {
       IO.print(this) // expect: Outer
 
       class Inner {
+        construct new() {}
+
         method {
           IO.print(this) // expect: Inner
         }