Separate tagging from collector

The collector now has an abstract interface onto the embedder.  The
embedder has to supply some functionality, such as tracing and
forwarding.  This is a pretty big change in terms of lines but it's
supposed to have no functional or performance change.

gc-forwarding.h

@@ -0,0 +1,20 @@
+#ifndef GC_FORWARDING_H
+#define GC_FORWARDING_H
+
+#include <stdint.h>
+
+enum gc_forwarding_state {
+  GC_FORWARDING_STATE_FORWARDED,
+  GC_FORWARDING_STATE_BUSY,
+  GC_FORWARDING_STATE_ACQUIRED,
+  GC_FORWARDING_STATE_NOT_FORWARDED,
+  GC_FORWARDING_STATE_ABORTED
+};
+
+struct gc_atomic_forward {
+  void *object;
+  uintptr_t data;
+  enum gc_forwarding_state state;
+};
+
+#endif // GC_FORWARDING_H