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.