Kernel: Add a LoopbackAdapter for talking to yourself via 127.0.0.1.

Choosing adapter for transmit is done by adapter_for_route_to(IPv4Address).
This is just hard-coded logic right now but can be expanded to support a
proper routing table.

Also start moving kernel networking code into Kernel/Net/.

Kernel/Net/Routing.h

@@ -0,0 +1,5 @@
+#pragma once
+
+#include <Kernel/NetworkAdapter.h>
+
+NetworkAdapter* adapter_for_route_to(const IPv4Address&);