VERY WIP - update for ocaml 5.0

[TyOverby]

This pull-request contains the diff necessary to get malfunction to compile (and pass all tests) in an ocaml 5.0 opam switch

This patch is bad for the following reasons:

• it becomes less portable by removing some ifdefs
• functions and types necessary for the switch functor invocation were implemented blindly
• values for new Pfield constructor args were basically picked at random

Please don't merge this, but maybe it's an ok starting-point for a real attempt