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/IPv4Packet.h

@@ -0,0 +1,58 @@
+#pragma once
+
+#include <AK/Types.h>
+#include <Kernel/IPv4Address.h>
+
+struct IPv4Protocol {
+enum {
+    ICMP = 1,
+};
+};
+
+class [[gnu::packed]] IPv4Packet {
+public:
+    byte version() const { return (m_version_and_ihl >> 4) & 0xf; }
+    void set_version(byte version) { m_version_and_ihl = (m_version_and_ihl & 0x0f) | (version << 4); }
+
+    byte internet_header_length() const { return m_version_and_ihl & 0xf; }
+    void set_internet_header_length(byte ihl) { m_version_and_ihl = (m_version_and_ihl & 0xf0) | (ihl & 0x0f); }
+
+    word length() const { return ntohs(m_length); }
+    void set_length(word length) { m_length = htons(length); }
+
+    word ident() const { return ntohs(m_ident); }
+    void set_ident(word ident) { m_ident = htons(ident); }
+
+    byte ttl() const { return m_ttl; }
+    void set_ttl(byte ttl) { m_ttl = ttl; }
+
+    byte protocol() const { return m_protocol; }
+    void set_protocol(byte protocol) { m_protocol = protocol; }
+
+    word checksum() const { return ntohs(m_checksum); }
+    void set_checksum(word checksum) { m_checksum = htons(checksum); }
+
+    const IPv4Address& source() const { return m_source; }
+    void set_source(const IPv4Address& address) { m_source = address; }
+
+    const IPv4Address& destination() const { return m_destination; }
+    void set_destination(const IPv4Address& address) { m_destination = address; }
+
+    void* payload() { return m_payload; }
+    const void* payload() const { return m_payload; }
+
+private:
+    byte m_version_and_ihl;
+    byte m_dscp_and_ecn;
+    word m_length;
+    word m_ident;
+    word m_flags_and_fragment;
+    byte m_ttl;
+    byte m_protocol;
+    word m_checksum;
+    IPv4Address m_source;
+    IPv4Address m_destination;
+    byte m_payload[0];
+};
+
+static_assert(sizeof(IPv4Packet) == 20);