extract udp into its own static library

include/udp.h

@@ -1,45 +0,0 @@
-#pragma once
-#include <cstdint>
-#include <span>
-#include "eth.h"
-#include "ipv4.h"
-#include "span_writer.h"
-
-namespace udp {
-
-struct __attribute__((packed)) header {
-    uint16_t src_port;
-    uint16_t dst_port;
-    uint16_t length;
-    uint16_t checksum;
-};
-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,
-             uint16_t src_port, uint16_t dst_port,
-             size_t payload_len, uint8_t ttl = 64) {
-    auto* u = buf.template prepend<header>();
-    u->src_port = src_port;
-    u->dst_port = dst_port;
-    u->length = __builtin_bswap16(sizeof(header) + payload_len);
-    u->checksum = 0;
-    ipv4::prepend(buf, dst_mac, src_mac, src_ip, dst_ip, 17, sizeof(header) + payload_len, ttl);
-}
-
-void init();
-void handle(std::span<const uint8_t> frame, span_writer& tx);
-
-using port_handler = void (*)(std::span<const uint8_t> payload, const address& from);
-void register_port(uint16_t port_be, port_handler fn);
-
-} // namespace udp