Fd_by_descr is a table of the open Fd.ts, indexed by file descriptor number.
Fd_by_descr
Fd.t
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.
Raw_fd.t
Fd
add_exn t fd fails if the file descriptor for fd is already in t.
add_exn t fd
fd
t