CHANGES:

• S.union and S.merge could invalidate the invariant:
  foreach k \in m . m[k] = (key, value) /\ k = key
  which could lead to assertion fail in find
• The signatures uses semi-explicit polymorphism with a record type:
  • S.equal : { f : 'a key -> 'a -> 'a -> bool } -> t -> t -> bool
  • S.merge : { f : 'a key -> 'a option -> 'a option -> 'a option } -> t -> t -> t
  • S.union : { f : 'a key -> 'a -> 'a -> 'a option } -> t -> t -> t
  • new function S.map : { f : 'a key -> 'a -> 'a } -> t -> t
• Interface duplication for "bindings" and "value" were removed:
  S.findb, S.getb, S.addb, S.addb_unless_bound no longer exist,
  use S.find, S.get, S.add, S.add_unless_bound instead.
• The pretty-printer S.pp was removed, and K.pp is no longer required! S.pp is:
  let pp ppf = M.iter (fun (M.B (k, v)) -> Fmt.pf ppf (K.pp k) v)
• no more Fmt dependency
• added some initial tests