This `Let_syntax` allows you to write expressions like

            let open Incr.Let_syntax in
            let%map_open some_incr = watch some_variable
            and another_incr = ...
            and ...
            in
            ...expression involving some_incr, another_incr, etc...

Note that this is less efficient than using `map3`, `map4`, etc., as the latter produces fewer intermediate nodes.

`val return : 'a ‑> 'a t`
`val (>>|) : 'a t ‑> ('a ‑> 'b) ‑> 'b t`
`val (>>=) : 'a t ‑> ('a ‑> 'b t) ‑> 'b t`
`module Let_syntax : sig ... end`