[review] revisit apply_member_pointer

[badair]

apply_member_pointer seems to be trying to do too much. I can get a
pointer to member data of any type, unless that data member is itself
a pointer to member or a pointer to function, in which case I get
something else entirely. Additionally, the description doesn't mention
that PMDs also get special handling.

from http://lists.boost.org/Archives/boost/2017/04/234173.php

[badair]

Is it useful at all? Should it be split into multiple traits, or removed entirely? What was the rationale for the current design?