Add clear function that clears the linked list.

include/ll.h

@@ -109,5 +109,12 @@ void* ll_pophead(ll* ll);
  */
 void* ll_poptail(ll* ll);
 
+/**
+ * Removes all elements from the list, freeing the underlying memory
+ * but keeping the data intact,
+ * @param ll the linked list to clear.
+ */
+void ll_clear(ll* ll);
+
 
 #endif