Script_HasNoEffect returns TRUE if the script definitely has no player-
visible effect, or FALSE if it may/does have a player-visible effect.

Opcodes have been manually tagged with whether they abort if they would
have a player-visible effect, and the natives and specials have been
manually tagged with whether they definitely have no player-visible
effect or not.

Using these, we're able to execute scripts until they either exit with
no effect, or would possibly have an effect. This allows us to:
1. Not run on frame map scripts or triggers if they would have no
   effect.
2. Support arbitrary control flow in trainer scripts. The trainer does
   not see the player if the script has no effect, and the trainer will
   use whichever trainerbattle command is branched to.
3. Support arbitrary scripts in trainer scripts. cant_see and
   cant_see_if_* commands have been introduced so that scripts are able
   to do something when the player interacts with the trainer even if
   that trainer wouldn't see them.