fix day 12 performance regerssion

caused by a change to Grid.at_e that would eagerly format an exception
string even if the function never throws an exception.

src/grid.ml

@@ -27,8 +27,9 @@ let at grid point =
 ;;
 
 let at_e (grid : 'a t) (point : Vec2.t) =
-  Option.get_exn_or (Format.sprintf "point %a out of bounds!" Vec2.pp point)
-  @@ at grid point
+  match at grid point with
+  | Some point -> point
+  | None -> failwith @@ Format.sprintf "point %a out of bounds!" Vec2.pp point
 ;;
 
 let set_e grid point item =