Item ordering

Currently, the linkme macro accepts an usize sort key. Although it is not documented, I assume this controls the ordering of the elements in the final slice. I was planing to use this to guarantee a deterministic ordering of entries between builds by supplying a hash of the item as the sort key. Otherwise, I end up with different orderings between release and debug builds. Unfortunately, the sort key is limited to a maximum of 9999, which is too small to avoid hash collisions.

It would be neat to have more control over the ordering of items.