Fd_by_descr is a table of the open Fd.ts, indexed by file descriptor number.
In this interface, we use Raw_fd.t rather than Fd.t to avoid a dependency cycle,
because the Fd module can't be defined yet.
include sig ... endval sexp_of_t : t ‑> Sexplib.Sexp.tval create : num_file_descrs:int ‑> tval add : t ‑> Raw_fd.t ‑> unit Core.Or_error.tval mem : t ‑> Import.File_descr.t ‑> boolval find : t ‑> Import.File_descr.t ‑> Raw_fd.t optionval find_exn : t ‑> Import.File_descr.t ‑> Raw_fd.t