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

firmware/include/dispatch.h

@@ -6,17 +6,17 @@
 #include <span>
 #include "wire.h"
 #include "timer_queue.h"
+#include "net.h"
 
 struct responder {
     uint32_t message_id;
-    std::function<void(std::span<const uint8_t>)> send;
+    send_fn send;
 
     template <typename T>
     void respond(const T& msg) const {
-        uint8_t buf[1024];
-        span_writer out(buf, sizeof(buf));
-        auto r = encode_response_into(out, message_id, msg);
-        if (r) send({buf, *r});
+        send([&](span_writer& out) {
+            return encode_response_into(out, message_id, msg);
+        });
     }
 };