Support ocaml 5.2

[cristianoc]

Fixes #196

[cknitt]

@cristianoc Thanks! Unfortunately I now get the errors

File "src/Noalloc.ml", line 67, characters 4-20:
Error: The constructor Tpat_var expects 3 argument(s),
       but is applied here to 2 argument(s)
File "src/Arnold.ml", line 518, characters 41-56:
Error: The constructor Tpat_var expects 3 argument(s),
       but is applied here to 2 argument(s)
File "src/DeadCommon.ml", line 339, characters 8-48:
Error: The constructor Tpat_var expects 3 argument(s),
       but is applied here to 2 argument(s)
File "src/Exception.ml", line 481, characters 6-40:
Error: The constructor Tpat_var expects 3 argument(s),
       but is applied here to 2 argument(s)

[cristianoc]

@cknitt check again now

[cknitt]

Thanks!
And sorry, we pushed fixes more or less simultaneously.
5.2 CI actually does work once the .opam file is updated.