SignedSubmissionIndices: StorageDescriptor<
    [],
    [
        { minimal_stake: bigint; sum_stake: bigint; sum_stake_squared: bigint },
        number,
        number,
    ][],
    false,
    never,
>

A sorted, bounded vector of (score, block_number, index), where each index points to a value in SignedSubmissions.

We never need to process more than a single signed submission at a time. Signed submissions can be quite large, so we're willing to pay the cost of multiple database accesses to access them one at a time instead of reading and decoding all of them at once.