Non-blocking, Async-friendly print functions

This module overrides everything in the Sys module that might block.

to_string t returns a pretty sexp of the representation of t

Fd_by_descr is a table of the open Fd.ts, indexed by file descriptor number.

File_descr_watcher provides an API for interacting with select-like functionality for watching a set of file descriptors to see if they are available for reading or writing.

The In_thread module has functions for interaction between the Async world and other (kernel) threads.

thread_safe_interrupt t causes read_fd t to become ready for reading.

Read_write is like Dirpair, except "buy/sell" has been changed to "read/write".

A reader lets one do buffered input from a file descriptor.

Threading model:

shutdown ?force status initiates shutdown, which runs all the at_shutdown functions, waits for them to finish, and then exits with the supplied status.

handle ?stop signals ~f runs f on each signal in signals that is delivered, stopping when stop becomes determined.

A signal manager keeps track of a set of signals to be managed and the signal handlers for them.

The Thread_safe module has functions that are safe to call from threads outside async.

Unix_syscalls provides an interface to many of the functions in OCaml's standard Unix module.

Writer provides an interface to writing strings that batches together writes into a single underlying write system call.