There is a quirk related to ordering: we store accounts in such an order that
is expected by multi-signature verification script. This was done to
speed up transaction/block signing which is done quite frequently in
tests. This commit allows to provide accounts in any order and to
extract a single signer from multi-signer based on this order.
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>
