More formatting with ocp-indent

[mjambon]

This extends #344. I indented all the ml and mli files with ocp-indent, using the usual config (.ocp-indent file) that I've been using in ocaml-tree-sitter and spacegrep among others.

ocp-indent, unlike reformatting tools like ocamlformat or refmt, will only touch the indentation of existing lines. The current config doesn't even indent the contents of comments, for maximum freedom. Benefits:

• In emacs, tuareg-mode will use it instead of its own mechanism
• It's usable from vim as well (?)
• We can run it as a pre-commit hook (not in this PR but I can set this up)
• Everyone ends up using the same indentation style
• It won't modify the code generated by ocaml-tree-sitter significantly or at all, making maintenance/merges easier

[mjambon]

I know this is one is weird. I filed an issue. I might fix it myself. See OCamlPro/ocp-indent#312