Drop Lwt in favour of effect handlers

[dhil]

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OCaml 5.00 is due to be released later this year. This release brings effect handlers to OCaml, and thus a native mechanism for programming with control. In the long term we should replace uses of Lwt in our code base with effect handlers, such that we can get rid of the pseudo-monadic programming style and instead use good ol' direct-style. This will also provide a good opportunity to rethink and possibly redesign parts of the Links process model.