arm: async echo app

arm/uart.h

@@ -1,7 +1,22 @@
 #pragma once
 
+#include <span>
+#include <string_view>
+
 #include "xuartlite.h"
 
 extern XUartLite* uart0;
 
 void InitUarts();
+
+// blocking
+void UartSend(std::span<const std::byte> data);
+inline void UartSend(std::string_view s) {
+    return UartSend(std::as_bytes(std::span{s.data(), s.size()}));
+}
+void UartSendCrash(std::span<const std::byte> data);
+inline void UartSendCrash(std::string_view s) {
+    return UartSendCrash(std::as_bytes(std::span{s.data(), s.size()}));
+}
+
+void HandleUartTxFromIsr(void*, unsigned int transmitted);