Eliminate std::function: fn-pointer callbacks, per-test test_state structs, udp.cpp with link-time udp::client::handler, udp::address

firmware/include/udp.h

@@ -1,7 +1,10 @@
 #pragma once
 #include <cstdint>
+#include <span>
 #include "eth.h"
 #include "ipv4.h"
+#include "net.h"
+#include "span_writer.h"
 
 namespace udp {
 
@@ -13,6 +16,14 @@ struct __attribute__((packed)) header {
 };
 static_assert(sizeof(header) == 8);
 
+struct address {
+    // mac is carried here until we grow an ARP cache; once we do, a
+    // destination mac comes from resolving ip and this field goes away.
+    eth::mac_addr mac;
+    ipv4::ip4_addr ip;
+    uint16_t port;
+};
+
 template <typename Buf>
 void prepend(Buf& buf, const eth::mac_addr& dst_mac, const eth::mac_addr& src_mac,
              ipv4::ip4_addr src_ip, ipv4::ip4_addr dst_ip,
@@ -26,4 +37,12 @@ void prepend(Buf& buf, const eth::mac_addr& dst_mac, const eth::mac_addr& src_ma
     ipv4::prepend(buf, dst_mac, src_mac, src_ip, dst_ip, 17, sizeof(header) + payload_len, ttl);
 }
 
+void handle(std::span<const uint8_t> frame, span_writer& tx);
+
+namespace client {
+// Defined by the higher layer (dispatch) to receive decoded UDP payloads
+// addressed to PICOMAP_PORT_BE. Resolved at link time.
+void handler(std::span<const uint8_t> payload, const address& from);
+} // namespace client
+
 } // namespace udp