Split Yield and Preempt into separate scheduling functions.

This switch makes the logic for each much easier to parse.

zion/scheduler/scheduler.h

@@ -12,6 +12,13 @@ void InitScheduler();
 // Enables the scheduler such that processes will yield on ticks.
 void EnableScheduler();
 
+// Preempts the current thread and flags it as runnable in the queue.
+// Generally used by the timer to move to the next timeslice.
+void Preempt();
+
+// Current thread yields and is not rescheduled until some external process
+// adds it.
+// Used when a thread blocks or exits.
 void Yield();
 
 // Scheduler will take ownership