`network.Address` structure in most cases created once and used read-only.
Replace `AddressFromString` function with `Address.FromString` method with
the same purpose and implementation. Make all libraries to work with value.
Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>
Also use neo-go private key wrapper where possible, as it
already has methods for (un)marshaling.
Signed-off-by: Evgenii Stratonikov <evgeniy@nspcc.ru>
