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/CMakeLists.txt

@@ -0,0 +1,11 @@
+add_library(arp STATIC arp.cpp)
+
+target_include_directories(arp PUBLIC ${CMAKE_CURRENT_SOURCE_DIR})
+
+target_compile_options(arp PRIVATE -Wall -Wextra -Wno-unused-parameter)
+
+target_link_libraries(arp PUBLIC
+    util
+    eth
+    ipv4
+)