Currently, due to a chain of omnivorous templated functions and constructor calls, co_return is much more lax with types than plain return. In particular, co_return can implicitly call explicit constructors, which is counter-intuitive.

We have seen bugs because of it. They include silent casting of enums such as raft::errc to cluster::errc just by index. Or strong types such as model::offset and kafka::offset cast one to another and/or to the underlying type.

This change is to avoid it. I understand it should be discussed with Seastar devs, PRing it here to gather internal opinions first.