add `cpo_signature_t<cpo, Args...>` type trait that computes the overload function type available to static_cast.

[atomgalaxy]

Lewis:

What about if we had a metafunction that allowed us to query cpo_signature_t<some_cpo, argetypelist...> and this resolves to a function signature that you can then static_cast<cpo_signature_t<some_cpo, T, U, V>>(some_cpo) to get the function pointer for an invocation some_cpo with arg types T, U and V.