tuple (get): Implement N3670.

tuple (get): Implement N3670.

	* include/std/tuple (get): Implement N3670.
	* include/std/utility (get): Likewise.
	* testsuite/20_util/pair/get.cc: Move to ...
	* testsuite/20_util/pair/astuple/get.cc: Here.
	* testsuite/20_util/pair/astuple/astuple.cc: New.
	* testsuite/20_util/pair/astuple/constexpr_get.cc: New.
	* testsuite/20_util/pair/astuple/constexpr_get_by_type.cc: New.
	* testsuite/20_util/pair/astuple/get_by_type.cc: New.
	* testsuite/20_util/pair/astuple/get_by_type_neg.cc: New.
	* testsuite/20_util/pair/astuple/get_neg.cc: New.
	* testsuite/20_util/tuple/element_access/constexpr_get_by_type.cc: New.
	* testsuite/20_util/tuple/element_access/get2_by_type.cc: New.
	* testsuite/20_util/tuple/element_access/get_by_type.cc: New.

From-SVN: r199082