Implement Serialize::visit_newtype_{struct,variant}
Implement Serialize::visit_newtype_{struct,variant}

These functions allow a serializer to not wrap a newtyped value
inside of a tuple or struct.

alexcrichton/toml-rs,badboy/toml-rs,GuillaumeGomez/toml-rs,mseri/toml-rs,trlim/toml-rs