Signers are very important for notary checks and keeping/passing an additional
copy of them is very inconvenient. Exposing them from invoker makes them
available in actors too.
Signed-off-by: Roman Khimov <roman@nspcc.ru>
Everywhere including examples, external interop APIs, bindings generators
code and in other valuable places. A couple of `interface{}` usages are
intentionally left in the CHANGELOG.md, documentation and tests.
