Add Env.update

src/env.ml

@@ -77,3 +77,6 @@ let diff x y =
     | Some _ -> None
     | None -> vx)
   |> make
+
+let update t ~var ~f =
+  make (Map.update t.vars var ~f)

src/env.mli

@@ -23,4 +23,7 @@ val add : t -> var:Var.t -> value:string -> t
 
 val diff : t -> t -> t
 
+val update : t -> var:string -> f:(string option -> string option) -> t
+
 val sexp_of_t : t -> Sexp.t
+