OCaml 5.3 support

[kit-ty-kate]

Currently irmin fails on OCaml 5.3 with:

#=== ERROR while compiling irmin.3.9.0 ========================================#
# context              2.4.0~alpha1~dev | linux/x86_64 | ocaml-variants.5.3.0+trunk | file:///home/opam/opam-repository
# path                 ~/.opam/5.3/.opam-switch/build/irmin.3.9.0
# command              ~/.opam/5.3/bin/dune build -p irmin -j 1
# exit-code            1
# env-file             ~/.opam/log/irmin-20-bc7cf4.env
# output-file          ~/.opam/log/irmin-20-bc7cf4.out
### output ###
# (cd _build/default && .ppx/851ddbf37b8928057d14b915d5ceebb6/ppx.exe --lib Type --cookie 'library-name="irmin"' -o src/irmin/node_intf.pp.ml --impl src/irmin/node_intf.ml -corrected-suffix .ppx-corrected -diff-cmd - -dump-ast)
# File "src/irmin/node_intf.ml", line 127, characters 7-13:
# 127 |   type effect := expected_depth:int -> node_key -> t option
#              ^^^^^^
# Error: Syntax error

[art-w]

Ooh great catch, thanks a ton! We should publish a new release soon with the fix :)