Remove neo-go dependency

[fyfyrchik]

It is a circular dependency and sometimes lead to problems (I had some problems GOPROXY=direct after cleaning the cache)
We use only 2 packages from there:
neo-go/pkg/io: to serialize example messages. They are redefined in neo-go (protocol uses interfaces), so example messages can use any serialization scheme (json, gob, raw encoding/binary helpers, whatever).
neo-go/pkg/util: for util.Uint* numbers. For these generics could be used.

IMO example messages are still good, to use in tests or simulation.
Another option is embed this library to neo-go, but this is more costly.

[roman-khimov]

Duplicates #2. I'll reopen it, but it was never a problem to me.