initial GC commit

include/picrin/gc.h

@@ -1,10 +1,16 @@
 #ifndef GC_H__
 #define GC_H__
 
+enum pic_gc_mark {
+  PIC_GC_MARK,
+  PIC_GC_UNMARK
+};
+
 union header {
   struct {
     union header *ptr;
     size_t size;
+    enum pic_gc_mark mark;
   } s;
   long alignment;   /* force alignment for headers to be allocated at the size of long */
 };