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/Scheduler.cpp

@@ -5,6 +5,7 @@
 #include <Kernel/Profiling.h>
 #include <Kernel/RTC.h>
 #include <Kernel/Scheduler.h>
+#include <Kernel/TimerQueue.h>
 
 //#define LOG_EVERY_CONTEXT_SWITCH
 //#define SCHEDULER_DEBUG
@@ -579,6 +580,8 @@ void Scheduler::timer_tick(RegisterDump& regs)
         }
     }
 
+    TimerQueue::the().fire();
+
     if (current->tick())
         return;