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>
