Sys module that might block.
      Fd.t is a wrapper around a Unix file descriptor, with additional information
about the kind of file descriptor and logic to ensure that we don't use a file
descriptor that has been closed, or close a file descriptor that is in use.
      Fd_by_descr is a table of the open Fd.ts, indexed by file descriptor number.
      File_descr_watcher_intf.S provides an API for for watching a set of file descriptors
to see if they are ready for reading or writing.
      In_thread module has functions for interaction between the Async world and other
(kernel) threads.
      Read_write is like Dirpair, except "buy/sell" has been changed to "read/write".
      Reader is Async's main API for buffered input from a file descriptor.
      Thread_safe module has functions that are safe to call from threads outside
Async.
      Async.Pipe.
      Unix_syscalls provides an interface to many of the functions in OCaml's standard
Unix module.