Start building the scheduler for Windows

Building derivations is a lot harder, but the downloading goals is
portable enough.

The "common channel" code is due to Volth. I wonder if there is a way we
can factor it out into separate functions / files to avoid some
within-function CPP.

Co-authored-by: volth <volth@volth.com>

src/libutil/windows/windows-async-pipe.hh

@@ -0,0 +1,20 @@
+#pragma once
+///@file
+
+#include "file-descriptor.hh"
+
+namespace nix::windows {
+
+class AsyncPipe
+{
+public:
+    AutoCloseFD writeSide, readSide;
+    OVERLAPPED overlapped;
+    DWORD got;
+    std::vector<unsigned char> buffer;
+
+    void createAsyncPipe(HANDLE iocp);
+    void close();
+};
+
+}