GC Pinning

We can root objects, but from what I can tell we can't pin to prevent them from being moved by the GC between collections.
My use case for this is wanting to make it easier to integrate with async libraries like libuv, currently if I want to write haxe bytes to a file through libuv I need to copy the data into a non GC staging buffer and write that otherwise the GC could move the haxe object mid write.
I've not really dug around the GC before but I can see a commented out `IMMIX_ALLOC_IS_PINNED` define which is not referenced anywhere else, was this functionality planned or existed in a previous version?

Thanks.