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/ICMP.h

@@ -0,0 +1,51 @@
+#pragma once
+
+#include <Kernel/MACAddress.h>
+#include <Kernel/IPv4Packet.h>
+#include <Kernel/NetworkOrdered.h>
+
+struct ICMPType {
+enum {
+    EchoReply = 0,
+    EchoRequest = 8,
+};
+};
+
+class [[gnu::packed]] ICMPHeader {
+public:
+    ICMPHeader() { }
+    ~ICMPHeader() { }
+
+    byte type() const { return m_type; }
+    void set_type(byte b) { m_type = b; }
+
+    byte code() const { return m_code; }
+    void set_code(byte b) { m_code = b; }
+
+    word checksum() const { return ntohs(m_checksum); }
+    void set_checksum(word w) { m_checksum = htons(w); }
+
+    const void* payload() const { return &m_payload[0]; }
+    void* payload() { return &m_payload[0]; }
+
+private:
+    byte m_type { 0 };
+    byte m_code { 0 };
+    word m_checksum { 0 };
+    dword m_rest_of_header { 0 };
+    byte m_payload[0];
+};
+
+static_assert(sizeof(ICMPHeader) == 8);
+
+struct [[gnu::packed]] IPv4ICMPPacket {
+    IPv4Packet ipv4_packet;
+    ICMPHeader icmp_header;
+};
+
+struct [[gnu::packed]] ICMPEchoPacket {
+    ICMPHeader header;
+    NetworkOrdered<word> identifier;
+    NetworkOrdered<word> sequence_number;
+    byte payload[];
+};