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
• https://github.com/ocaml/opam-repository/pull/14351

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

[XVilka]

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

[XVilka]

Seems it just boils down to crafting a new release: http://opam.ocaml.org/packages/lambdoc/