Use OCaml 4.08's let operators for monadic code

[Anurag Soni]

OCaml 4.08 added monadic let operators -> ocaml/ocaml#1947

Lwt merged support for let* and let+ as part of the following PRs

• ocsigen/lwt#775
• ocsigen/lwt#776

We can use let* and let+ where we use Lwt's >>= and >|= right now. This should make some parts of the implementation easier to read as the monadic code will read similar to regular let bindings.