refactor: init and destroy, alloc now first ensures cap

its better usability and is better that it ensures capacity in alloc
like that is what is supposed to be like damn, why i did it like that

include/arena.h

@@ -46,7 +46,7 @@ typedef struct {
 } ArenaSizeResult;
 
 ArenaErr arena_init(Arena **arena, size_t capacity);
-void arena_destroy(Arena **arena);
+ArenaErr arena_destroy(Arena **arena);
 
 ArenaPointer arena_alloc(Arena *arena, size_t size, size_t alignment);
 ArenaPointer arena_push(Arena *arena, void *data, size_t size, size_t alignment);