Module Observer.Update

on_update_exn t ~f calls f after the current stabilization and after each subsequent stabilization in which t changes, until disallow_future_use t is called. f will be called at most once per stabilization. Here is a state diagram for the allowable sequences of Update.t's that can be supplied to a particular f:

   /-----------------------------------------------------\
   |                 /                                   |
   |                 |                                   v
  Start ------> Initialized ------> Changed ------> Invalidated
                                      ^  |
                                      \--/

on_update_exn raises if disallow_future_use t was previously called.