Move /src to /subprojects

This will facilitate breaking up Nix into multiple packages for each
component with Meson.

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

@@ -0,0 +1,27 @@
+#pragma once
+///@file
+
+#include "file-descriptor.hh"
+
+namespace nix::windows {
+
+/***
+ * An "async pipe" is a pipe that supports I/O Completion Ports so
+ * multiple pipes can be listened too.
+ *
+ * Unfortunately, only named pipes support that on windows, so we use
+ * those with randomized temp file names.
+ */
+class AsyncPipe
+{
+public:
+    AutoCloseFD writeSide, readSide;
+    OVERLAPPED overlapped;
+    DWORD got;
+    std::vector<unsigned char> buffer;
+
+    void createAsyncPipe(HANDLE iocp);
+    void close();
+};
+
+}