Allow `SignOption` to set 2 parameters:
1. Default signature scheme, which is used in case scheme is
unspecified.
2. Restrict scheme option which also checks that scheme is either
unspecified or equal to the restricted scheme. This is only used
for verification and is necessary because some of the signatures
are used in smart-contracts.
Also provide signature struct to sign/verify functions in helpers.
The constant names differ a bit from those in API because of linter
complaints.
Signed-off-by: Evgenii Stratonikov <evgeniy@nspcc.ru>
