Zero-copy ethernet TX: encode directly into prepend_buffer, add info/test skills

firmware/include/net.h

@@ -13,8 +13,11 @@ struct net_state {
     ipv4::ip4_addr ip;
 };
 
+using encode_fn = std::function<msgpack::result<size_t>(span_writer&)>;
+using send_fn = std::function<void(encode_fn)>;
+
 using net_handler = std::function<void(std::span<const uint8_t> payload,
-                                       std::function<void(std::span<const uint8_t>)> send)>;
+                                       send_fn send)>;
 
 using net_frame_callback = std::function<bool(std::span<const uint8_t> frame)>;