fix: add cancellation support for Windows socket connect

- Add impl reference and do_cancel() override to connect_op struct
- Implement connect_op::do_cancel() calling CancelIoEx on the socket
- Initialize conn_ member with socket impl reference in constructor

Author: vinniefalco

src/detail/win_iocp_sockets.cpp

@@ -111,9 +111,22 @@ do_cancel() noexcept
     }
 }
 
+void
+connect_op::
+do_cancel() noexcept
+{
+    if (impl.is_open())
+    {
+        ::CancelIoEx(
+            reinterpret_cast<HANDLE>(impl.native_handle()),
+            this);
+    }
+}
+
 win_socket_impl::
 win_socket_impl(win_iocp_sockets& svc) noexcept
     : svc_(svc)
+    , conn_(*this)
     , rd_(*this)
     , wr_(*this)
 {

src/detail/win_iocp_sockets.hpp

@@ -40,6 +40,11 @@ class win_acceptor_impl;
 /** Connect operation state. */
 struct connect_op : overlapped_op
 {
+    win_socket_impl& impl;
+
+    explicit connect_op(win_socket_impl& impl_) noexcept : impl(impl_) {}
+
+    void do_cancel() noexcept override;
 };
 
 /** Read operation state with buffer descriptors. */