add contraint on ocf.0.8.0: ocaml < 5.3.0

[zoggy]

No description provided.

[zoggy]

It seems ok to me.

[mseri]

Fails with

#=== ERROR while compiling ocf.0.8.0 ==========================================#
# context              2.4.0~alpha1~dev | linux/x86_64 | ocaml-base-compiler.5.3.0~alpha1 | file:///home/opam/opam-repository
# path                 ~/.opam/5.3~alpha1/.opam-switch/build/ocf.0.8.0
# command              ~/.opam/opam-init/hooks/sandbox.sh build dune build -p ocf -j 31 --promote-install-files=false @install
# exit-code            1
# env-file             ~/.opam/log/ocf-7-911278.env
# output-file          ~/.opam/log/ocf-7-911278.out
### output ###
# (cd _build/default && /home/opam/.opam/5.3~alpha1/bin/ocamlc.opt -g -w -40 -bin-annot -w -6-7-9-10-27-32-33-34-35-36-50-52 -no-strict-sequence -g -bin-annot -bin-annot-occurrences -I lib/.ocf.objs/byte -I /home/opam/.opam/5.3~alpha1/lib/seq -I /home/opam/.opam/5.3~alpha1/lib/yojson -intf-suffix .ml -no-alias-deps -o lib/.ocf.objs/byte/ocf.cmo -c -impl lib/ocf.ml)
# File "lib/ocf.ml", line 215, characters 16-33:
# 215 |     { wrapper = Obj.magic wrapper ;
#                       ^^^^^^^^^^^^^^^^^
# Error: This field value has type "'b wrapper" which is less general than
#          "'a. 'a wrapper"
# (cd _build/default && /home/opam/.opam/5.3~alpha1/bin/ocamlopt.opt -g -w -40 -bin-annot -w -6-7-9-10-27-32-33-34-35-36-50-52 -no-strict-sequence -g -I lib/.ocf.objs/byte -I lib/.ocf.objs/native -I /home/opam/.opam/5.3~alpha1/lib/seq -I /home/opam/.opam/5.3~alpha1/lib/yojson -intf-suffix .ml -no-alias-deps -o lib/.ocf.objs/native/ocf.cmx -c -impl lib/ocf.ml)
# File "lib/ocf.ml", line 215, characters 16-33:
# 215 |     { wrapper = Obj.magic wrapper ;
#                       ^^^^^^^^^^^^^^^^^
# Error: This field value has type "'b wrapper" which is less general than
#          "'a. 'a wrapper"