arraylist length function added

include/acl/array.h

@@ -1,6 +1,7 @@
 #ifndef array_h
 #define array_h
 #include <stddef.h>
+size_t acl_arraylist_len(void *arraylist);
 void acl_arraylist_free(void *arraylist);
 void* acl_arraylist_remove(void *arraylist_void, size_t index);
 void* acl_arraylist_create(size_t array_size, size_t sizeof_one_element);