OCaml 5.4 support

[kit-ty-kate]

The latest version of num fails to build when using OCaml 5.4:

#=== ERROR while compiling num.1.5-1 ==========================================#
# context     2.4.0~alpha2 | linux/x86_64 | ocaml-base-compiler.5.4.0~alpha1 | https://opam.ocaml.org#584630e7a7e27e3cf56158696a3fe94623a0cf4f
# path        ~/.opam/default/.opam-switch/build/num.1.5-1
# command     ~/.opam/opam-init/hooks/sandbox.sh build make PROFILE=release opam-modern
# exit-code   2
# env-file    ~/.opam/log/num-20663-eff40a.env
# output-file ~/.opam/log/num-20663-eff40a.out
### output ###
# [...]
# ocamlc -w +a-4-9-41-42-44-45-48  -bin-annot -g -safe-string -strict-sequence -strict-formats -I +compiler-libs -c num_top_printers.mli
# ocamlc -w +a-4-9-41-42-44-45-48  -bin-annot -g -safe-string -strict-sequence -strict-formats -I +compiler-libs -c num_top_printers.ml
# ocamlc -w +a-4-9-41-42-44-45-48  -bin-annot -g -safe-string -strict-sequence -strict-formats -I +compiler-libs -c num_top.mli
# ocamlc -w +a-4-9-41-42-44-45-48  -bin-annot -g -safe-string -strict-sequence -strict-formats -I +compiler-libs -c num_top.ml
# File "num_top.ml", line 28, characters 36-61:
# 28 |                               (Ldot(Lident "Num_top_printers", s))
#                                          ^^^^^^^^^^^^^^^^^^^^^^^^^
# Error: This expression should not be a constructor, the expected type is
#        Longident.t Location.loc
# make[1]: *** [Makefile:86: num_top.cmo] Error 2
# make[1]: Leaving directory '/home/opam-health-check/.opam/default/.opam-switch/build/num.1.5-1/src'
# make: *** [Makefile:2: all] Error 2

[xavierleroy]

Ah, the misery of using compiler APIs... There seems to be a simple, backward-compatible fix, see #46.