Support OCaml 5.5

[patricoferris]

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[NathanReb]

Thanks!

[NathanReb]

We need to bump the upper bound in our dune-project/opam files to get a bit of feedback from the CI.

We'll also need to update ast/supported_version/supported_version.ml, astlib/config/gen.ml and to add identity migrations and the relevant ast_505.ml.

I'm happy include these extra steps in a separate PR.

[NathanReb]

Did you use our new gencopy to generate the migrations? how did it work out?

[NathanReb]

Looks good to me! Still curious to hear your feedback on the gencopy workflow to see if we can improve it somehow.

I guess one thing we could change is that it should probably dump a comment at the beginning of the file, hinting that it was initially generated with that tool and later modified. Having that comment point to the dev/README.md would help maintainers discover it if they're not aware of its existence.

[patricoferris]

Thanks for the review, @NathanReb ! I think this already might be out of date for some new changes in 5.5, but I'd rather get this is in and fix those up later in a separate PR.

Re: gencopy -- yes! It was extremely useful. Some notes from me:

• It can be a bit awkward to actually use the tool itself because you start by switching to a compiler that is not supported so you cannot build gencopy itself. Of course, you can switch back to an earlier compiler, so perhaps just some clear documentation on that part.
• The changes from 5.4 to 5.5 included updated names for record fields, this sounds like something gencopy might be able to deal with? It seemed to choose one name or the other consistently for the migrations which meant one was always wrong. In practice though, this is a simple enough post-edit to the files.
• We now also have two constants -- one in Asttypes and the other in the AST itself, however the generate identical names. I think the Asttypes one does not actually need to be generated so I just deleted it.