Properly constrain detail::copy optimization

[localspook]

Resolves #4470.

See linked issue for a description of the problem. This PR constrains detail::copy to only use insert when the underlying container provides it.

[vitaut]

Thanks for the PR. I suggest merging has_back_insert_iterator_container_append and has_back_insert_iterator_container_insert_at_end into a single trait, something like has_compatible_container that checks the required container properties.

[localspook]

Could you elaborate? We have one copy overload that does .append(), one that does .insert(), and one that uses the iterator normally, and I don't see how I could express all of those with just one trait.

[vitaut]

Ah, you are right. We do need separate traits.