message: add database-key-index type
diff: @@ -410,6 +410,17 @@ where
  }
  }
  +/// An integer that uniquely identifies a particular query instance within the
  +/// database. Used to track dependencies between queries. Fully ordered and
  +/// equatable but those orderings are arbitrary, and meant to be used only for
  +/// inserting into maps and the like.
  +#[derive(Copy, Clone, PartialEq, Eq, PartialOrd, Ord, Hash, Debug)]
  +pub struct DatabaseKeyIndex {
  + group_index: u16,
  + query_index: u16,
  + key_index: u32,
  +}
  +
  /// Trait implements by all of the "special types" associated with
  /// each of your queries.
  ///
  