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This applies orders hierarchically. Unfortunately, the interfaces had to be
changed significantly to get this to work.

For example `[OrderByValue{}, SortByKey{}]` *used* to re-sort by key, throwing
away the value sort. Now, this acts like a normal database and `SortByKey` will
only sort within equivalent values.

None of our actual datastores support any other types and the cast noise gets
really annoying after a while.