Multisig Revamp for StructuredData/AppendableData

General
2

Part 1 - Performance:

Currently signatures is a separate field, and other fields are unsorted vectors. This leads to following inefficiencies:

  1. O(n^2) check for duplicates.

  2. O(n^2) check for verification. Each false hit entails a throw-away signature verification.

This can be optimised if we use a HashMap instead of a vector and integrate signatures field with owners field. The ShareableData would now take the form:

ShareableData {
    //  .... other fields ...
    current_owners: HashMap<Owner, Option<Signature>>,
    prev_owners: HashMap<Owner, Option<Signature>>,
}

Those opting to sign would put the signature mapped to their ownership instead of in a different field. Data to sign would be same as previously:

sign(other_fields + current_owners.keys() + previous_owners.keys());

This eliminates an O(n^2) here and turns this from O(n^2) to O(n).