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

firmware/lib/ipv4.cpp

@@ -1,7 +1,7 @@
 #include "ipv4.h"
 #include "icmp.h"
 #include "igmp.h"
-#include "net.h"
+#include "udp.h"
 #include "parse_buffer.h"
 
 namespace ipv4 {
@@ -47,7 +47,7 @@ void handle(std::span<const uint8_t> frame, span_writer& tx,
     case 17:
         if (!ip_match(ip->dst, our_ip, subnet_broadcast))
             return;
-        net_handle_udp(frame, tx);
+        udp::handle(frame, tx);
         break;
     }
 }