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);

src/array.c

@@ -54,3 +54,7 @@ void* acl_arraylist_remove(void *arraylist_void, size_t index) {
 	}
 	return arraylist;
 }
+
+size_t acl_arraylist_len(void *arraylist) {
+	return ((union arraylist_meta*)arraylist - 1)->len;
+}