Fix lifetime issues for Face::halfedges(), Cell::halffaces().
Currently, the following API-usage is straight-forward and clean but unfortunately wrong and dangerous:
for (const auto heh: mesh.face(fh).halfedges()) { ... }
This is because the temporary Face returned by face() reaches the
end of its lifetime, but halfedges() returned a reference to a vector that
lives inside that object. Reference lifetime extension does not apply
here.
This change splits behaviour so that in rvalue context (such as temporaries), we move out the vector instead of returning a reference to it.