Implement C++17 string searchers.

Implement C++17 string searchers.

	* include/std/functional: (unordered_map, vector): New includes
	in C++17 mode.
	(array, bits/stl_algo.h): Likewise.
	(default_searcher, __boyer_moore_map_base): New.
	(__boyer_moore_array_base, __is_std_equal_to): Likewise.
	(__boyer_moore_base_t, boyer_moore_searcher): Likewise.
	(boyer_moore_horspool_searcher, make_default_searcher): Likewise.
	(make_boyer_moore_searcher): Likewise.
	(make_boyer_moore_horspool_searcher): Likewise.
	* testsuite/20_util/function_objects/searchers.cc: New.

From-SVN: r240093