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-ref.h

@@ -0,0 +1,37 @@
+#ifndef GC_REF_H
+#define GC_REF_H
+
+#include "gc-assert.h"
+
+#include <stdint.h>
+
+struct gc_ref {
+  uintptr_t value;
+};
+
+static inline struct gc_ref gc_ref(uintptr_t value) {
+  return (struct gc_ref){value};
+}
+static inline uintptr_t gc_ref_value(struct gc_ref ref) {
+  return ref.value;
+}
+
+static inline struct gc_ref gc_ref_null(void) {
+  return gc_ref(0);
+}
+static inline int gc_ref_is_heap_object(struct gc_ref ref) {
+  return ref.value != 0;
+}
+static inline struct gc_ref gc_ref_from_heap_object_or_null(void *obj) {
+  return gc_ref((uintptr_t) obj);
+}
+static inline struct gc_ref gc_ref_from_heap_object(void *obj) {
+  GC_ASSERT(obj);
+  return gc_ref_from_heap_object_or_null(obj);
+}
+static inline void* gc_ref_heap_object(struct gc_ref ref) {
+  GC_ASSERT(gc_ref_is_heap_object(ref));
+  return (void *) gc_ref_value(ref);
+}
+
+#endif // GC_REF_H