A proposal to add a utility class to represent optional objects (Revision 4)

Added std::optional at Bristol 2013; reverted at Chicago 2013 to the Library Fundamentals TS. Applied in N3690/N3691 with several editorial reorganizations; later removed from the C++14 WP at Chicago (LWG Motion 9, moved to Library Fundamentals TS).; reverted at 2013-09 Chicago; moved to Library Fundamentals TS (std::optional returned in C++17 as P0220R1).