Kernel: Add kernel-level timer queue (heavily based on @juliusf's work)

PR #591 defines the rationale for kernel-level timers. They're most
immediately useful for TCP retransmission, but will most likely see use
in many other areas as well.

Kernel/TimerQueue.h

@@ -0,0 +1,48 @@
+#pragma once
+
+#include <AK/Function.h>
+#include <AK/NonnullOwnPtr.h>
+#include <AK/OwnPtr.h>
+#include <AK/SinglyLinkedList.h>
+#include <Kernel/Arch/i386/PIT.h>
+
+struct Timer {
+    u64 id;
+    u64 expires;
+    Function<void()> callback;
+    bool operator<(const Timer& rhs) const
+    {
+        return expires < rhs.expires;
+    }
+    bool operator>(const Timer& rhs) const
+    {
+        return expires > rhs.expires;
+    }
+    bool operator==(const Timer& rhs) const
+    {
+        return id == rhs.id;
+    }
+};
+
+enum TimeUnit {
+    MS = TICKS_PER_SECOND / 1000,
+    S = TICKS_PER_SECOND,
+    M = TICKS_PER_SECOND * 60
+};
+
+class TimerQueue {
+public:
+    static TimerQueue& the();
+
+    u64 add_timer(NonnullOwnPtr<Timer>&&);
+    u64 add_timer(u64 duration, TimeUnit, Function<void()>&& callback);
+    bool cancel_timer(u64 id);
+    void fire();
+
+private:
+    void update_next_timer_due();
+
+    u64 m_next_timer_due { 0 };
+    u64 m_timer_id_count { 0 };
+    SinglyLinkedList<NonnullOwnPtr<Timer>> m_timer_queue;
+};