extend source and target calculi with observable tick effects + add invariant checks for relation preservation and target arity/shape

src/gen.ml

@@ -85,6 +85,10 @@ let sample_terms ~count ~max_depth () =
                   Source.Fix ("loop", TInt, Source.Var "loop") ),
               Source.Int 0 );
       };
+      {
+        ty = TInt;
+        term = Source.Let ("_", Source.Tick ("alloc", Source.Int 0), Source.Int 1);
+      };
     ]
   in
   let rec fill acc i attempts =