Improve proving code (follow up on #3860)

[tomusdrw]

1. Add typedefs for storage types, to avoid Vec 🤢
   Remove requirement on Hash = H256, make Proposer return StorageChanges and Proof #3860 (comment)

2. Check if we can avoid clones of both keys and values when calculating delta:
   Remove requirement on Hash = H256, make Proposer return StorageChanges and Proof #3860 (comment)

[stanislav-tkach]

I have tried to deal with the second part of this issue several times, but now I'm thinking it isn't worth it. At least not until we are sure that we have a bottleneck there.