109.34.00

- Added `val Backtrace.get_opt : unit -> t option`.

  This is more convenient to use than `Backtrace.get`, which is an
  `Or_error.t`.

- Moved functions for dealing with finalizers into the `Gc.Expert` module.

  This was done to make people be very explicit and sure that they want
  finalizers, which are very hard to use because they essentially
  introduce multithreading semantics.

  One should typically use async finalizers.

- Eliminated the thread that had been used to sequentialize all finalizers.