Implement P0253R1, Fixing a design mistake in the searchers
	interface in Library Fundamentals.
	* include/std/functional: (utility): New include in C++17 mode.
	(default_searcher): Use a pair as return type, adjust the definition.
	(boyer_moore_searcher): Likewise.
	(boyer_moore_horspool_searcher): Likewise.
	* testsuite/20_util/function_objects/searchers.cc: Adjust.

From-SVN: r240094