[2023-06 LWG Motion 18] P0792R14 function_ref: a type-erased callable reference

[jensmaurer]

See also #6275

[JohelEGP]

There will be a merge conflict with #6313

• In the synopsis of <functional>, and
• at the beginning of the new subclause.

Care will be needed to ensure there's only one \indextext{function object!wrapper|)}.