Add todo axiom (#158)

Author: janmasrovira

Stdlib/Debug/Todo.juvix

@@ -0,0 +1,4 @@
+module Stdlib.Debug.Todo;
+
+--- Something that hasn't yet been implemented
+axiom todo {A : Type} : A;

Stdlib/Prelude.juvix

@@ -17,5 +17,6 @@ import Stdlib.Data.Result open public;
 import Stdlib.Data.Range open public;
 import Stdlib.Function open public;
 import Stdlib.System.IO open public;
+import Stdlib.Debug.Todo open public;
 
 import Stdlib.Trait open public;