IOCP support (again)

[PyXiion]

Well, there's a lot of work to do to do that.

Right now I need to write/rewrite these things:

• io_notifier_iocp
• pipe_t: rename it to wake_source or something like that, and implement it on Windows via PostQueuedCompletionStatus and custom completion keys
• poll_info & poll_op
• timer_handle:
  1. std::multimap/std::set with timers, and calculate dynamic timeout for GQCS(iocp, ..., timeout_ms).
     On new timer wake IOCP queue with PQCS(..., TIMER_WAKE_KEY, nullptr).
     • Pros: one thread, no thread pool
     • Cons: heap usage, possibly bad performance with many timers, maybe precision loss
  2. CreateThreadpoolTimer + PostQueuedCompletionStatus from the callback
     • Pros: OS will manage it itself, high precision, simpler code
     • Cons: overhead for short timers, thread pool used
  3. CreateWaitableTimer + dedicated waiter thread
     • Pros: super duper simple
     • Cons: extra thread and overall bad

[PyXiion]

I think I'll try to move all async operations into io_notifier to make the API consistent on both Windows & Unix

class io_notifier_epoll
{
  // ...

    // renamed poll_info
    auto watch(async_operation& pi) -> bool;
    
    // ...

    // Operations
    auto write_some(net::socket &socket, std::span<const std::byte> buffer) -> coro::task<std::pair<net::io_status, ssize_t>>;
    auto read_some(net::socket &socket, std::span<std::byte> buffer) -> coro::task<std::pair<net::io_status, ssize_t>>;
    
    // File I/O may look like this in the future
    // auto write_some(coro::file &file, std::span<const std::byte> buffer) -> coro::task<std::pair<io_status, ssize_t>>;
};