Add Scheduler wireframe.

Right now does nothing but has containing classes for process and thread
information.

zion/scheduler/thread.h

@@ -0,0 +1,23 @@
+#pragma once
+
+#include <stdint.h>
+
+// Forward decl due to cyclic dependency.
+class Process;
+
+class Thread {
+ public:
+  Thread(Process* proc, uint64_t thread_id) : process_(proc), id_(thread_id) {}
+
+  uint64_t tid() { return id_; };
+  uint64_t pid();
+
+  Process& process() { return *process_; }
+
+ private:
+  Process* process_;
+  uint64_t id_;
+
+  // Next task in queue for quick scheduling.
+  Thread* next_task_;
+};