Consider adding a way to retrieve parameter list without the class type

Opened to track comment in https://lists.boost.org/Archives/boost/2017/04/234374.php:

> I've a feeling that it might also be useful to provide an extractor that, for member pointers, returns just the arguments without the class.

Right now, `args_t` is `std::tuple`. It might be useful to provide a way of retrieving the argument list without the type of `*this`, such that one would get `std::tuple`.

Start with the args_t example in the issue and read the linked Boost mailing-list discussion to understand the proposed extractor semantics. Compare the existing member-pointer behavior with the desired tuple result; done means a decided API and corresponding implementation and tests, though no repository files or test paths are named.

Written by the indexing model from the issue text.