Refactor delta's vault and storage to use maps

[Mirko-von-Leipzig]

Refactor delta's vault and storage to use maps

[bobbinth]

I think we can take it a bit further in terms making deltas more "specialized". For example, AccountVaultDelta could look something like this:

pub struct AccountVaultDelta {
    fungible: BTreeMap<AccountId, i64>,
    non_fungible: BTreeMap<Digest, bool>,
}

I would also probably update serialization to work roughly as follows:

• First serialize fungible asset updates as (account_id, amount_delta) tuples.
• Then serialize added non-fungible assets.
• Then serialize removed non-fungible assets.

For AccountStorageDelta, I think it still may make sense to keep the simple slot updates separate from map updates. Specifically, it could look something like this:

pub struct AccountStorageDelta {
    slots: BTreeMap<u8, Word>,
    maps: BTreeMap<u8, StorageMapDelta>,
}

pub struct StorageMapDelta(BTreeMap<Digest, Word>);

For serialization, we could probably still keep something that is roughly similar to the current structure:

• First serialize updated slots (i.e., where the new slot value is non-empty word) as (slot_index, slot_value) tuples.
• Then serialize cleared slots (for this we only need to serialize slot indexes).
• Then serialize storage map deltas as:
  • First updated leaves as a list of (key, value) tuples.
  • Then serialize cleared leaves as a list of just keys.