Add a clear function for the vector.

include/vec.h

@@ -88,5 +88,11 @@ int vec_foreach(vec* vec, void* data, compare_func compare, foreach_func foreach
  * @return pointer to the value, or, if the index is out of bounds (or there is nothing there) NULL.
  */
 void* vec_index(vec* vec, int index);
+/**
+ * Clears the vector, removing all elements from it
+ * but not resizing it down.
+ * @param vec the vector to clear.
+ */
+void vec_clear(vec* vec);
 
 #endif