The AnnSortKey is used to keep track of the order of declarations for
printing when the container has split them apart.

This applies to HsValBinds and ClassDecl, ClsInstDecl.

When making modifications to the list of declarations, the new order
must be captured for when it must be printed. For each list of
declarations (binds and sigs for a HsValBind) we can just store the
list in order.

To recreate the list when printing, we must merge them, and this is
what the AnnSortKey records. It used to be indexed by SrcSpan, we now
simply index by a marker as to which list to take the next item from.