Picos_lwt_unixSourceDirect style Picos compatible interface to Lwt with Lwt_unix for OCaml 5.
run main is equivalent to calling run_fiber with a freshly created fiber and main wrapped to capture the result of main.
The optional forbid argument defaults to false and determines whether propagation of cancelation is initially allowed.
run_main main is equivalent to Lwt_main.run (run main).