Non-blocking, Async-friendly print functions

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

An 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.

thread_safe_interrupt t causes read_fd t to become ready for reading.

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

Reader is Async's main API for buffered input from a file descriptor.

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

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.

We override values from Core.Signal that we don't want people to use with Async.

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 is Async's main API for output to a file descriptor.