Enable ocamlformat

[smorimoto]

No description provided.

[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.

[aantron]

This is not a helpful comment, but I'm sad about this. Unlike most code, lwt.ml is carefully formatted to look good, and ocamlformat is introducing things such as this indentation.

[aantron]

To make it potentially somewhat helpful, in the past we applied ocp-indent (then the go-to formatting tool) to the almost the whole repo, but we avoided touching code that had gotten the full human attention.