OCaml 5.5: Support all Pexp_struct_items

[patricoferris]

See the related compiler PR: ocaml/ocaml#14040

Currently, we only migrate the previously supported structure items that could appear in a let-binding (let open etc.). Now, however, the compiler allows anything to appear in a local let-binding except additional let-bindings (let let ...) and includes (let include). Whereas the previous change had a 1:1 mapping from a 5.4 AST node to a 5.5 AST node, now we do not and will need to think of a suitable encoding.