eio

Merge pull request #822 from patricoferris/fiber-docs Fix docs for without_binding

Add Eio_unix.Stdenv.with_env for updating environments

Adds support for interacting with devices on macOS Block devices currently did not work on macOS, due to a bug that prevents poll(2) from working with block devices. This fixes that by creating a new thread that monitors block devices using select(2). The thread is created the first time a user computation attempts to read or write to a block devices. The implementation of the new select manager is in select.ml Additionally, the select manager also needed an eventfd system. The eventfd system from sched.ml was refactoring into a re-usable model that encapsulates some of the internal state in posix_eventfd.ml.