Remove `signature` and `util/signature` packages. Re-implement their
functionality in new `crypto` package. Generalize the approach of
digital signature computation and verification by adding `Signer` and
`PublicKey` primitives similar to standard `crypto` package. Support
already exising in protocol signature schemes.
Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>
This is done to prevent import cycles when `object` package needs any other
that requires `object.ID` or `object.Address`.
Signed-off-by: Pavel Karpy <carpawell@nspcc.ru>
