Draft: Everywhere: Add some support for OCaml 4.12

Context

This replaces !2504 (closed) but targets master instead of a release branch.

Note that it's only preparatory work towards moving to ocaml4.12, it doesn't change the version of OCaml the software is compiled against.

Manually testing the MR

Checklist

• N/A Document the interface of any function added or modified (see the coding guidelines)
• N/A Provide automatic testing (see the testing guide).
• N/A For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, the Development Version section of CHANGES.md for everything else).
• Select suitable reviewers using the Reviewers field below.