OCaml 5.3 support

[kit-ty-kate]

malfunction currently fails to build with the upcoming OCaml 5.3:

#=== ERROR while compiling malfunction.0.6 ====================================#
# context              2.3.0~alpha~dev | linux/x86_64 | ocaml-variants.5.3.0+trunk | file:///home/opam/opam-repository
# path                 ~/.opam/5.3/.opam-switch/build/malfunction.0.6
# command              ~/.opam/5.3/bin/dune build -p malfunction -j 1
# exit-code            1
# env-file             ~/.opam/log/malfunction-20-c96ea6.env
# output-file          ~/.opam/log/malfunction-20-c96ea6.out
### output ###
# (cd _build/default && /home/opam/.opam/5.3/bin/ocamlc.opt -w -40 -g -bin-annot -bin-annot-occurrences -I src/.malfunction.objs/byte -I /home/opam/.opam/5.3/lib/findlib -I /home/opam/.opam/5.3/lib/ocaml/compiler-libs -I /home/opam/.opam/5.3/lib/ocaml/dynlink -I /home/opam/.opam/5.3/lib/ocaml/str -I /home/opam/.opam/5.3/lib/ocaml/unix -I /home/opam/.opam/5.3/lib/zarith -intf-suffix .ml -no-alias-deps -o src/.malfunction.objs/byte/malfunction_compiler.cmo -c -impl src/malfunction_compiler.ml)
# File "src/malfunction_compiler.ml", line 774, characters 4-21:
# 774 |     Env.set_unit_name module_name;
#           ^^^^^^^^^^^^^^^^^
# Error: Unbound value "Env.set_unit_name"