This patch adds std::shift_left and std::shift_right as per P0769R2.  Alhough
these are STL-style algos, this patch places them in <bits/ranges_algo.h>
because they make use of some functions in the ranges namespace that are more
easily reachable from <bits/ranges_algo.h> than from <bits/stl_algo.h>, namely
ranges::next.  In order to place these algos in <bits/stl_algo.h>, we would need
to include <bits/range_access.h> from <bits/stl_algo.h> which would undesirably
increase the size of <bits/stl_algo.h>.

libstdc++-v3/ChangeLog:

	P0769R2 Add shift to <algorithm>
	* include/bits/ranges_algo.h (shift_left, shift_right): New.
	* testsuite/25_algorithms/shift_left/1.cc: New test.
	* testsuite/25_algorithms/shift_right/1.cc: New test.