Add Scheduler wireframe.

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

zion/scheduler/process.h

@@ -0,0 +1,32 @@
+#pragma once
+
+#include <stdint.h>
+
+// Forward decl due to cyclic dependency.
+class Thread;
+
+class Process {
+ public:
+  // Caller takes ownership of returned process.
+  static Process* RootProcess();
+  Process();
+
+  uint64_t id() { return id_; }
+
+  Thread* CreateThread();
+  Thread* GetThread(uint64_t tid);
+
+ private:
+  Process(uint64_t id, uint64_t cr3) : id_(id), cr3_(cr3) {}
+  uint64_t id_;
+  uint64_t cr3_;
+
+  uint64_t next_thread_id_ = 0;
+
+  // FIXME: Make a better data structure for this.
+  struct ThreadEntry {
+    Thread* thread;
+    ThreadEntry* next;
+  };
+  ThreadEntry* thread_list_front_;
+};