Enable ocamlformat

[smorimoto]

[MisterDA]

I'm in favour of using ocamlformat, for consistency and readability. You left out the files using cppo (e.g. lwt_unix.cppo.ml) because ocamlformat doesn't support files using preprocessing, right? Is there a way we could cheat and add a formatted version of these files to the PR? To ease git-blaming, I also suggest that we add a .git-blame-ignore-revs file, containing the commit id of the re-formatting. The file can then be used with git blame --ignore-revs-file .git-blame-ignore-revs. See some blog posts explaining the idea: Ignoring mass reformatting commits with git blame, Exclude linting & formatting commits when running Git blame.