This patch uses the additional provisions from LWG 4264, to avoid double
indirection when function_ref is constructed from function_ref with
compatible signature.
The details of compatible signatures follows the move_only_function
as described in r16-617-g708d40ff109c6e49d02b684a368571722a160af8.
This commit series implements double indirection avoidance for the
function_ref and it is based on my patch for function_ref implementation.
This patch is send as RFC instead of PATCH ready for review, as I have
noticed that performing this implementation is user-observable and not
allowed in the
