SignedSubmissionIndices: StorageDescriptor<[], [{
    minimal_stake: bigint;
    sum_stake: bigint;
    sum_stake_squared: bigint;
}, number, number], false>

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.