Make the behaviour of `committee.Single(n)` more predictable,
i.e be able to return every committee member.
Signed-off-by: Evgeniy Stratonikov <evgeniy@nspcc.ru>
Implementing a separate `Signer` interface is beneficial in multiple
ways:
1. Support both single and multiple transaction witnesses.
2. It should be easy to add contract signer this way.
Tests should use accounts created with `NewAccount` so hiding all
details doesn't seem to be an issue.
Signed-off-by: Evgeniy Stratonikov <evgeniy@nspcc.ru>
