Implement std::optional.

Implement std::optional.

	* include/Makefile.am: Add optional to exported headers.
	* include/Makefile.in: Likewise.
	* include/std/optional: New.
	* testsuite/20_util/optional/typedefs.cc: Likewise.
	* testsuite/20_util/optional/relops/2.cc: Likewise.
	* testsuite/20_util/optional/relops/3.cc: Likewise.
	* testsuite/20_util/optional/relops/4.cc: Likewise.
	* testsuite/20_util/optional/relops/5.cc: Likewise.
	* testsuite/20_util/optional/relops/1.cc: Likewise.
	* testsuite/20_util/optional/relops/6.cc: Likewise.
	* testsuite/20_util/optional/nullopt.cc: Likewise.
	* testsuite/20_util/optional/in_place.cc: Likewise.
	* testsuite/20_util/optional/make_optional.cc: Likewise.
	* testsuite/20_util/optional/assignment/2.cc: Likewise.
	* testsuite/20_util/optional/assignment/3.cc: Likewise.
	* testsuite/20_util/optional/assignment/4.cc: Likewise.
	* testsuite/20_util/optional/assignment/5.cc: Likewise.
	* testsuite/20_util/optional/assignment/1.cc: Likewise.
	* testsuite/20_util/optional/assignment/6.cc: Likewise.
	* testsuite/20_util/optional/cons/value_neg.cc: Likewise.
	* testsuite/20_util/optional/cons/default.cc: Likewise.
	* testsuite/20_util/optional/cons/move.cc: Likewise.
	* testsuite/20_util/optional/cons/value.cc: Likewise.
	* testsuite/20_util/optional/cons/copy.cc: Likewise.
	* testsuite/20_util/optional/requirements.cc: Likewise.
	* testsuite/20_util/optional/observers/2.cc: Likewise.
	* testsuite/20_util/optional/observers/3.cc: Likewise.
	* testsuite/20_util/optional/observers/4.cc: Likewise.
	* testsuite/20_util/optional/observers/5.cc: Likewise.
	* testsuite/20_util/optional/observers/1.cc: Likewise.
	* testsuite/20_util/optional/constexpr/relops/2.cc: Likewise.
	* testsuite/20_util/optional/constexpr/relops/3.cc: Likewise.
	* testsuite/20_util/optional/constexpr/relops/4.cc: Likewise.
	* testsuite/20_util/optional/constexpr/relops/5.cc: Likewise.
	* testsuite/20_util/optional/constexpr/relops/1.cc: Likewise.
	* testsuite/20_util/optional/constexpr/relops/6.cc: Likewise.
	* testsuite/20_util/optional/constexpr/nullopt.cc: Likewise.
	* testsuite/20_util/optional/constexpr/in_place.cc: Likewise.
	* testsuite/20_util/optional/constexpr/make_optional.cc: Likewise.
	* testsuite/20_util/optional/constexpr/cons/default.cc: Likewise.
	* testsuite/20_util/optional/constexpr/cons/value.cc: Likewise.
	* testsuite/20_util/optional/constexpr/observers/2.cc: Likewise.
	* testsuite/20_util/optional/constexpr/observers/3.cc: Likewise.
	* testsuite/20_util/optional/constexpr/observers/4.cc: Likewise.
	* testsuite/20_util/optional/constexpr/observers/5.cc: Likewise.
	* testsuite/20_util/optional/constexpr/observers/1.cc: Likewise.
	* testsuite/20_util/optional/swap/1.cc: Likewise.

From-SVN: r238197