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.
Turns out, nothing should be changed in our implementation because
nil-check is done implicitly by type casts or type switches.
Signed-off-by: Evgeniy Stratonikov <evgeniy@nspcc.ru>