Add EventLoop#wait_readable, #wait_writable methods methods

[ysbaddaden]

Adds two methods to the Crystal::EventLoop interfaces to wait on a file descriptor or socket readiness (read or write) without doing the actual read or write operations, which can be delegated to an external library.

This provides a semi-public interface to work around #15374:

file_descriptor = IO::FileDescriptor.new(LibC.some_fd, blocking: false)

event_loop = Crystal::EventLoop.current
event_loop.wait_readable(file_descriptor)
LibC.do_something(fd)

This is implemented for the polling event loops (epoll, kqueue) as well as libevent since these were straightforward.

Windows is left unimplemented. It might be implemented with WSAPoll running in a thread or ProcessSocketNotifications to associate sockets to a completion port. See Winsock socket state notifications for more details.

Related to RFC #0007 and RFC #0009.