Deterministic Pay-to-script-hash multi-signature addresses through public key sorting.

Defines how to sort the keys behind a shared, multi-key address, so the same keys always give the same address.

Some addresses need signatures from several keys to spend. Listing the same keys in a different order gave a different address, making these shared accounts hard to recognise. Sorting the public keys in a fixed way means compatible wallets always produce one address for a given set of keys.