A GC alarm calls a user function after the end of each major GC cycle.

`type t`````
`include sig ... end`
`val sexp_of_t : t ‑> Base.Sexp.t`
`val create : (unit ‑> unit) ‑> t`

`create f` arranges for `f` to be called after the end of each major GC cycle, starting with the current cycle or the next one. `f` will be run in the monitor that `create` was called in.

`val delete : t ‑> unit`

`delete t` will stop the calls to the function associated with `t`. Calling `delete t` again has no effect.