add Event abstraction in Util

src/util/Event.ml

@@ -0,0 +1,14 @@
+type 'a handler = 'a -> unit
+type 'a t = { h: 'a handler Vec.t } [@@unboxed]
+
+let nop_handler_ = ignore
+
+module Emitter = struct
+  type nonrec 'a t = 'a t
+
+  let emit (self : _ t) x : unit = Vec.iter self.h ~f:(fun h -> h x)
+  let create () : _ t = { h = Vec.make 3 nop_handler_ }
+end
+
+let on self f = Vec.push self.h f
+let of_emitter x = x

src/util/Event.mli

@@ -0,0 +1,12 @@
+type 'a t
+(** An event emitting values of type ['a] *)
+
+module Emitter : sig
+  type 'a t
+
+  val emit : 'a t -> 'a -> unit
+  val create : unit -> 'a t
+end
+
+val on : 'a t -> ('a -> unit) -> unit
+val of_emitter : 'a Emitter.t -> 'a t