use gadts for layer, clean code

src/layer.mli

@@ -1,21 +1,37 @@
-type t
+type _ t =
+  | Basic : Jv.t -> [> `Basic ] t
+  | Geojson : Jv.t -> [> `Geojson ] t
+  | Marker : Jv.t -> [> `Marker ] t
+  | Tile : Jv.t -> [> `Tile ] t
 
-val add_to : Map.t -> t -> unit
+(** Basic layers *)
 
-val remove : t -> unit
+val add_to : Map.t -> _ t -> unit
 
-val remove_from : Map.t -> t -> unit
+val remove : _ t -> unit
 
-val bind_popup : Brr.El.t -> t -> unit
+val remove_from : Map.t -> _ t -> unit
 
-val unbind_popup : t -> unit
+val bind_popup : Brr.El.t -> _ t -> unit
 
-val open_popup : t -> unit
+val unbind_popup : _ t -> unit
 
-val close_popup : t -> unit
+val open_popup : _ t -> unit
 
-val get_popup : t -> Popup.t
+val close_popup : _ t -> unit
 
-val of_jv_t : Jv.t -> t
+val get_popup : _ t -> Popup.t
 
-val to_jv_t : t -> Jv.t
+val to_jv : _ t -> Jv.t
+
+(** Geojson layers *)
+
+val create_geojson : ?options:Jv.t -> Jv.t -> [ `Geojson ] t
+
+(** Marker layers *)
+
+val create_marker : Latlng.t -> [ `Marker ] t
+
+(** Tile layers *)
+
+val create_tile_osm : string option -> [ `Tile ] t