libstdc++: Reduce <functional> inclusion to <stl_algobase.h>

Move the std::search definition from stl_algo.h to stl_algobase.h and use
the later in <functional>.

For consistency also move std::__parallel::search and associated helpers from
<parallel/stl_algo.h> to <parallel/stl_algobase.h> so that std::__parallel::search
is accessible along with std::search.

libstdc++-v3/ChangeLog:

	* include/bits/stl_algo.h
	(std::__search, std::search(_FwdIt1, _FwdIt1, _FwdIt2, _FwdIt2, _BinPred)): Move...
	* include/bits/stl_algobase.h: ...here.
	* include/std/functional: Replace <stl_algo.h> include by <stl_algobase.h>.
	* include/parallel/algo.h (std::__parallel::search<_FIt1, _FIt2, _BinaryPred>)
	(std::__parallel::__search_switch<_FIt1, _FIt2, _BinaryPred, _ItTag1, _ItTag2>):
	Move...
	* include/parallel/algobase.h: ...here.
	* include/experimental/functional: Remove <bits/stl_algo.h> and <parallel/algorithm>
	includes. Include <bits/stl_algobase.h>.