Add a swap function that swaps contents of references.

include/refcount.h

@@ -83,6 +83,10 @@ void libab_ref_free(libab_ref* ref);
  * Copies this reference, thereby increasing the reference count.
  */
 void libab_ref_copy(const libab_ref* ref, libab_ref* into);
+/**
+ * Swaps the contents of two references.
+ */
+void libab_ref_swap(libab_ref* left, libab_ref* right);
 /**
  * Function that can be passed in to refcount to simply use free
  * when the refcount reaches 0.