mbv: add async app, sorta works

mbv/apps/async/uart_async.h

@@ -0,0 +1,15 @@
+#pragma once
+
+#include <span>
+#include <string_view>
+
+#include "async.h"
+#include "buffer.h"
+
+async::task<buffer> UartRead(int size);
+async::task<uint8_t> UartReadLoop();
+async::task<> UartWrite(std::span<const std::byte> data);
+inline async::task<> UartWrite(std::string_view s) {
+    co_await UartWrite(std::as_bytes(std::span{s.data(), s.size()}));
+}
+async::task<> UartWriteLoop(async::gimme<std::span<const std::byte>>& data);