Kernel: Start adding IPv4 support, starting with ICMP echo messages.

This doesn't work correctly yet, but it's getting nice enough to commit.

Kernel/NetworkOrdered.h

@@ -0,0 +1,48 @@
+#pragma once
+
+#include <AK/Types.h>
+
+template<typename T>
+T convert_between_host_and_network(T host_value)
+{
+    if constexpr (sizeof(T) == 4) {
+        auto* s = (byte*)&host_value;
+        return (dword)(s[0] << 24 | s[1] << 16 | s[2] << 8 | s[3]);
+    }
+    if constexpr (sizeof(T) == 2) {
+        auto* s = (byte*)&host_value;
+        return (word)(s[0] << 8 | s[1]);
+    }
+    if constexpr (sizeof(T) == 1)
+        return host_value;
+}
+
+template<typename T>
+class [[gnu::packed]] NetworkOrdered {
+public:
+    NetworkOrdered()
+        : m_network_value(0)
+    {
+    }
+
+    NetworkOrdered(const T& host_value)
+        : m_network_value(convert_between_host_and_network(host_value))
+    {
+    }
+
+    NetworkOrdered(const NetworkOrdered& other)
+        : m_network_value(other.m_network_value)
+    {
+    }
+
+    NetworkOrdered& operator=(const NetworkOrdered& other)
+    {
+        m_network_value = other.m_network_value;
+        return *this;
+    }
+
+    operator T() const { return convert_between_host_and_network(m_network_value); }
+
+private:
+    T m_network_value;
+};