Refactor and remove copy mark bits.

Refactor code GC realted code to be in a GC folder.

Remove copy mark bits by using pointer changing instead.

Enable concurrent sweeping of system weaks.

Fix non concurrent GC plan.

Change-Id: I9c71478be27d21a75f8a4e6af6faabe896e5e263
diff --git a/src/runtime.cc b/src/runtime.cc
index 3444f78..dc37ff2 100644
--- a/src/runtime.cc
+++ b/src/runtime.cc
@@ -41,7 +41,7 @@
 #include "signal_catcher.h"
 #include "signal_set.h"
 #include "sirt_ref.h"
-#include "space.h"
+#include "gc/space.h"
 #include "thread.h"
 #include "thread_list.h"
 #include "trace.h"