Update code to be compatible with Ocaml 4.13.1

[yashrk]

Hi.

Is this library still maintained?

I updated the code so it can be built with more recent OCaml compiler now.

PR also includes results of dune fmt, because sometimes fixes make the formatting completely broken (for example, in a long chain of binds in Lwt-related code) and to do dune fmt was the simplest solution to repair it.

I just fixed the compilation errors quickly, because the design solutions aren't clear for me (for example, should read be a part of the interface of Connection_lwt or not — some examples are still using it). Sometimes probably there is better solution, we can discuss it.

Also I still can't understand how to run the test suite without TravisCI.

Thank you for your great work.