Add Univ_map.singleton

Signed-off-by: Jeremie Dimino <jeremie@dimino.org>

src/stdune/univ_map.ml

@@ -71,3 +71,4 @@ let find_exn t key =
     let eq = Key.eq key' key in
     Eq.cast eq v
 
+let singleton key v = Int.Map.singleton (Key.id key) (Binding.T (key, v))

src/stdune/univ_map.mli

@@ -17,3 +17,4 @@ val add      : t -> 'a Key.t -> 'a -> t
 val remove   : t -> 'a Key.t -> t
 val find     : t -> 'a Key.t -> 'a option
 val find_exn : t -> 'a Key.t -> 'a
+val singleton : 'a Key.t -> 'a -> t