Windows: Event loop based on IOCP

[straight-shoota]

This patch implements the event loop for windows, based on IOCP. It follows up on #10784 which introduced overlapped operations.

It consists of:

• The original implementation from WIP: Win32 event loop #11776, rebased on current master
• @wonderix' refactoring to move WSAOVERLAPPED on the heap and keep it allocated until the cancellation is completed.
• Some minor code enhancements (no functional changes)
• Skip unix socket specs on win32, so now you can successfully run (most) socket specs via .\bin\crystal spec .\spec\std\socket\

Cudos to @wonderix for digging into this and finding the flaw in how we used the Win32 API, with thanks to @yxhuvud and @HertzDevil for helping out along the way (see discussion in #11776). Thanks to @neatorobito who laid the groundwork in #9957, as well as @kubo and @cfsamson for their input in #6957. 🙇

Digging through all these discussions shows it's been a long road.

But it looks like we now have a pretty much functional event loop on Windows 🚀

Closes #9957
Closes #11776

[wonderix]

In my first implementation, I enqueued the OverlappedOperation inside the start method. For this, I needed a separate state STARTED for this. This is no longer required since the enqueuing only happens when the operation is cancelled.
I think STARTED could be removed.

[straight-shoota]

Yes, I believe there can be some more refactoring of the state. But I'd leave it as is for now and do that in a follow-up later. The current code is working and I'd prefer to get it into master soon and then work on improving it further.

[Sija]

They're here - #12146

[beta-ziliani]

Two improvements for the future:

• The pending spec.
• A bit of comments to help understand the needed pointer arithmetic (or a patch that avoids it, if there can be such thing).

But let's get it rolling 🚀 Thanks ppl for solving this important issue! 🙇

[beta-ziliani]

FTR in theory nothing is preventing this to work, all of the ingredients are there, but it's still failing.

[beta-ziliani]

(PS: I'm adding to the 1.5 milestone even when we're in the freeze period because it doesn't affect any of the supported platforms)

[shayneoneill]

Guys, I know this is a work page (and a closed one), but my dudes take a moment to pat yourselves on the pat, because you beautiful maniacs deserve that pat on the back.
The event loop is a huge leap forward, and personally every attempt I've made in the part to try working with the windows port was thwarted by it. It works! Your all amazing!

We're now one step closer to being able to use my favorite language, anywhere its needed. And thats a glorious thing.

Thankyou!