3. Add a new attribute - e.g. @rvalue ref - which inserts a temporary variable for rvalues so that they can be passed by ref, and it works with both templated and non-templated functions.
We could also somehow use 'return ref' for that purpose, or we could get rid of the pointless 'const scope' alias 'in' and use 'in ref' as a new attribute. But somehow I would prefer 'auto ref', it's a more descriptive term.
