Eliminate stored value serialization differences in native contracts

[roman-khimov]

After #894 we'll have all the state properly stored in the native contract's data, but our serialization format will still differ from the one used by C# implementation and we can't ignore the difference here because of state changes (neo-project/neo#1383) that are expected to happen after preview2. If our serialization will be different we'll never match the state root hash with reference implementation and won't be able to interoperate.

The key difference is that C# implementation uses VM data type wrappers around state values, so that structures are serialized as VM structures, integers as VM integers, etc. We already have VM items serialization so it shouldn't be a problem to make proper wrappers for us.

[roman-khimov]

I think it'd be more appropriate to do this after #912 with StackItem move out of the vm.