109.53.00

- Added `Linux_ext.Epoll.close`.
- Added `Weak_hashtbl` module, moved from `Async`.

  It had only been in `Async` to use `Async`'s finalizers.  The move
  to `Core` exposes a bit more with respect to finalization so that
  one can still implement `Async.Weak_hashtbl`, as well as do other
  things (e.g. use `Weak_hashtbl` in `Incremental`, which does not use
  `Async`).

  Simplified the implementation of `Weak_hashtbl` to eliminate "entry
  ids".  They were only used to avoid removing a table entry that was
  in use.  But there is a more direct way to test for that -- just
  check if the weak is `None` or `Some`.
- Added an autoload file for utop
- Disabled warning 40 in corebuild