Add #wait_readable and #wait_writable to IO::FileDescriptor and Socket

[ysbaddaden]

Follow up to #15376 that exposes the #wait_readable and #wait_writable methods to IO::FileDescriptor and Socket directly.

I'm not sure this is the actual way to go. This is mostly an example to get started discussing on how to go forward for a more public API for #15374 and the ideas from RFC #0007:

• Bulk events without fibers
• Integration with select

[will]

I don't have an opinion on what the public API should be here, but just wanted to tag @stakach here since I know this is a problem with libssh2 spider-gazelle/ssh2.cr#22

[stakach]

wait_readable / wait_writable work for me
should be possible to implement on Windows too for the socket side of things, it's not possible with files on windows
https://gist.github.com/stakach/4d68e19e14feeaf251686ae6aaa54b77

Not that I use libssh2 on windows but it might be useful for someone and implementing the socket checks for windows would make it possible