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/check_jni.cc b/src/check_jni.cc
index 5146039..e7590d3 100644
--- a/src/check_jni.cc
+++ b/src/check_jni.cc
@@ -23,7 +23,7 @@
 #include "logging.h"
 #include "object_utils.h"
 #include "scoped_thread_state_change.h"
-#include "space.h"
+#include "gc/space.h"
 #include "thread.h"
 #include "runtime.h"