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/LoopbackAdapter.h

@@ -0,0 +1,16 @@
+#pragma once
+
+#include <Kernel/NetworkAdapter.h>
+
+class LoopbackAdapter final : public NetworkAdapter {
+public:
+    static LoopbackAdapter& the();
+
+    virtual ~LoopbackAdapter() override;
+
+    virtual void send_raw(const byte*, int) override;
+    virtual const char* class_name() const override { return "LoopbackAdapter"; }
+
+private:
+    LoopbackAdapter();
+};