No more default constructors.

Fixes #296.

example/import-module/cthulu.wren

@@ -1,3 +1,4 @@
 class Cthulu {
+  construct new() {}
   message { "Ph'nglui mglw'nafh Cthulhu R'lyeh wgah'nagl fhtagn" }
 }

example/import-module/lovecraft.wren

@@ -1,6 +1,7 @@
 import "cthulu" for Cthulu
 
 class Lovecraft {
+  construct new() {}
   say() { Cthulu.new().message }
 }