OCaml 5.4 support

[kit-ty-kate]

mdx currently fails on OCaml 5.4 with the following error message:

#=== ERROR while compiling mdx.2.5.0 ==========================================#
# context              2.4.0~beta1 | linux/x86_64 | ocaml-base-compiler.5.4.0~alpha1 | file:///home/opam/opam-repository
# path                 ~/.opam/5.4/.opam-switch/build/mdx.2.5.0
# command              ~/.opam/5.4/bin/dune build -p mdx -j 1 @install
# exit-code            1
# env-file             ~/.opam/log/mdx-14-22597a.env
# output-file          ~/.opam/log/mdx-14-22597a.out
### output ###
# (cd _build/default && /home/opam/.opam/5.4/bin/ocamlc.opt -w -40 -g -bin-annot -bin-annot-occurrences -I lib/top/.mdx_top.objs/byte -I /home/opam/.opam/5.4/lib/astring -I /home/opam/.opam/5.4/lib/camlp-streams -I /home/opam/.opam/5.4/lib/csexp -I /home/opam/.opam/5.4/lib/findlib -I /home/opam/.opam/5.4/lib/fmt -I /home/opam/.opam/5.4/lib/logs -I /home/opam/.opam/5.4/lib/ocaml-version -I /home/opam/.opam/5.4/lib/ocaml/compiler-libs -I /home/opam/.opam/5.4/lib/ocaml/str -I /home/opam/.opam/5.4/lib/ocaml/threads -I /home/opam/.opam/5.4/lib/ocaml/unix -I /home/opam/.opam/5.4/lib/re -I /home/opam/.opam/5.4/lib/result -I /home/opam/.opam/5.4/lib/seq -I lib/.mdx.objs/byte -I vendor/odoc-parser/src/.odoc_parser.objs/byte -no-alias-deps -open Mdx_top__ -o lib/top/.mdx_top.objs/byte/mdx_top__Compat_top.cmi -c -intf lib/top/compat_top.pp.mli)
# File "lib/top/compat_top.mli", line 11, characters 40-69:
# Error: Unbound type constructor Types.constructor_description

[jonludlam]

Fixed by #470

[kit-ty-kate]

Thanks a lot!