[review] apply_return needs a member function counterpart

> I would've expected a make_fn and make_mem_fn function, with which one could construct a type from a return type and an std::tuple of the arguments. That can be easily done manually but I would've expected that in this library.

from http://lists.boost.org/Archives/boost/2017/04/234190.php

Start by reading the existing apply_return implementation and related callable-construction APIs. Clarify the intended make_fn and make_mem_fn signatures from the linked request, then identify how return types and std::tuple arguments should map to each callable form. Done means the API exists with coverage for both free and member function cases.

Written by the indexing model from the issue text.