adds binding operators to the monads library

[ivg]

Instead of extending the Syntax module of the Monad.S and
Monad.S2 interface, which will break the interfaces, we add a new
module Let which includes binding operators for Monadic and
applicative interfaces (let*/and* and let+/and+
correspondingly).

That means that in order to be able to use the monadic operators for
the monad Example we need to open either Example or Example.Let
first, so we can write

open Example.Let

let example x y =
  let* r = compute x in
  let+ s = compute y in
  r + s

instead of the old style with fun,

open Example.Syntax

let example x y =
  compute x >>= fun r ->
  compute y >>| fun s ->
  r + s

Parallel binding is provided via the (and*) and (and+) operators,
so it is possible to write

open Example.Let

let example x y =
  let* r = compute x in
  and* s = compute y in
  Example.return (r + s)