Allow finalizers on custom blocks in the minor heap, fix pr 3612

This subsume the pull request https://github.com/ocaml/ocaml/pull/82

Note that this also make functions like Bigarray.Array1.sub a lot faster:

````
  for i = 0 to n do
    ignore(Array1.sub a 3 5)
  done
````
Is more than twice faster.
I am not sure this is the right place to add that kind of comment: it may become obsolete if sweep_slice evolve.