OCaml library : Ephemeron.K2.Bucket

`sig
type ('k1, 'k2, 'd) t
val make : unit -> ('k1, 'k2, 'd) Ephemeron.K2.Bucket.t
val add : ('k1, 'k2, 'd) Ephemeron.K2.Bucket.t -> 'k1 -> 'k2 -> 'd -> unit
val remove : ('k1, 'k2, 'd) Ephemeron.K2.Bucket.t -> 'k1 -> 'k2 -> unit
val find : ('k1, 'k2, 'd) Ephemeron.K2.Bucket.t -> 'k1 -> 'k2 -> 'd option
val length : ('k1, 'k2, 'd) Ephemeron.K2.Bucket.t -> int
val clear : ('k1, 'k2, 'd) Ephemeron.K2.Bucket.t -> unit
end`