util/async.c: Add async IO worker thread

Need this for IO4 USB emulation, might break it out and upstream it
to Capnhook at some point as well.

util/async.h

@@ -0,0 +1,24 @@
+#pragma once
+
+#include <windows.h>
+
+#include <stdbool.h>
+
+#include "hook/iohook.h"
+
+typedef HRESULT (*async_task_t)(void *ctx, struct irp *irp);
+
+struct async {
+    CRITICAL_SECTION lock;
+    CONDITION_VARIABLE pend;
+    CONDITION_VARIABLE avail;
+    HANDLE thread;
+    struct irp irp;
+    async_task_t task;
+    void *ctx;
+    bool stop;
+};
+
+void async_init(struct async *async, void *ctx);
+void async_fini(struct async *async);
+HRESULT async_submit(struct async *async, struct irp *irp, async_task_t task);