Add OCaml 5.4 support (WIP)

[shindere]

No description provided.

[glondu]

I had to apply the attached patch to make stdcompat compile with OCaml 5.4.0.
0002-Fix-OCaml-5.4.0-patch.patch

[tmcgilchrist]

@shindere I think you're only missing a change to stdcompat__format_s.mli.in to add out_width from @glondu patch. Then this change compiles fine with OCaml 5.4.

[shindere]

Thanks, tmcgilchrist. You are right, but do you realise that, so far, "supporting OCaml 5.4" did *not* just mean compile with it, but rather expose to users of older OCaml compilers all the features they had, were they using OCaml 5.4? This is why doing a release of libstdcompat is so hard, it's because you need to make allthe features provided in newer versions of the stdlib with older compilers. So for instance OCaml 5.4 adds immutable arrays, among many other things. The goal and very purpose of stdcompat is to make immutable arrays available to users of older OCaml compilers. We indeed now think about having two kind of releases of stdcompat, one kind which does compiler with a given compiler but does not necessarily expose all the features of the associated standard library (this would be a novelty) and one version that does both: compile with a certain version of the OCaml compiler _and_ backport the features of its associated standard library to older compilers. Am I making sense @tmcgilchrist? Do you understand what I mean now?