add function to convert string to CoreKey.key_code (for parsing/loading controls from files)

shell/input-state.sml

@@ -30,7 +30,9 @@ struct
     , jumpHeld = !(#jumpHeld state)
     }
 
-  fun actionToBool action = action = Input.PRESS
+  (* there are three action states reported by OS: PRESS, REPEAT and RELEASE. 
+   * If input is PRESS or REPEAT, then return true, or else return false. *)
+  fun actionToBool action = action <> Input.RELEASE
 
   fun handleKey (key, action) =
     case GlfwKeyMap.codeFromKey key of

shell/parse-controls.sml

@@ -0,0 +1 @@
+structure ParseControls = struct end