We have a lot of native contract types that are converted to stack items
before serialization, then deserialized as stack items and converted back to
regular structures. stackitem.Convertible allows to remove a lot of repetitive
io.Serializable code.
This also introduces to/from converter in testserdes which unfortunately
required to change util tests to avoid circular references.
We should store each GAS record as
{
Key: ContractID + RecordsPrefix + RecordID,
Value: RecordValue,
}
So don't use state.GASRecord for storing anymore. However, it's still
useful to cache GasRecord values by using state.GASRecords, because we
have to keep GASIndexPairs sorted by indexes.
