Migrate from ulex to sedlex

[XVilka]

Long story short - ulex is dying, mainstream depends on the camlp4, which is dead, fork depends on the camlp5, which seems dead too. This prevents from porting to more modern OCaml versions and dependencies versions.

See these links for more information:

• https://discuss.ocaml.org/t/camlp5-and-ocaml-4-08/3985/4
• Rename ulex-camlp5 into ulex ocaml/opam-repository#14351

Sedlex is an active maintained project by OCaml community: https://github.com/ocaml-community/sedlex

[hbbio]

Thanks for reporting. Opa is not actively maintained currently but maybe some new or previous contributors might want to have a look at that and globally compiling Opa with the modern OCaml stack.

[XVilka]

And the official Camlp4 deprecation announcement: https://discuss.ocaml.org/t/the-end-of-camlp4/4216