Sync lifetime names with `session` package. Add `AssertContainer` and
`AssertUser` methods which cover specific cases describe in protocol.
Increase test coverage.
Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>
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>
