extract arp into its own static library; add empty arp::init() called from dispatch_init to anchor the .o so the self-registration constructor links in

arp/arp.h

@@ -0,0 +1,25 @@
+#pragma once
+#include <span>
+#include "eth.h"
+#include "ipv4.h"
+#include "span_writer.h"
+
+namespace arp {
+
+struct __attribute__((packed)) header {
+    uint16_t htype;
+    uint16_t ptype;
+    uint8_t hlen;
+    uint8_t plen;
+    uint16_t oper;
+    eth::mac_addr sha;
+    ipv4::ip4_addr spa;
+    eth::mac_addr tha;
+    ipv4::ip4_addr tpa;
+};
+static_assert(sizeof(header) == 28);
+
+void init();
+void handle(std::span<const uint8_t> frame, span_writer& tx);
+
+} // namespace arp