vm: added noop instruction, various other improvements

lib/vm/types.ml

@@ -25,6 +25,7 @@ type instr =
   | Jump of int
   | JumpF of int (* jump if false. *)
   | End
+  | NOOP
 
 type vm_state = {
     mutable i : int;

lib/vm/vm.ml

@@ -57,5 +57,6 @@ and interpret state =
      (match (pop_one state) with
      | Nil -> state.i <- target
      | _ -> ()); interpret state 
-  | End -> ())
+  | End -> ()
+  | NOOP -> interpret state)