This patch implements the C++20 ranges overloads for the algorithms in
[algorithms].  Most of the algorithms were reimplemented, with each of their
implementations very closely following the existing implementation in
bits/stl_algo.h and bits/stl_algobase.h.  The reason for reimplementing most of
the algorithms instead of forwarding to their STL-style overload is because
forwarding cannot be conformantly and efficiently performed for algorithms that
operate on non-random-access iterators.  But algorithms that operate on random
access iterators can safely and efficiently be forwarded to the STL-style
implementation, and this patch does so for push_heap, pop_heap, make_heap,
sort_heap, sort, stable_sort, nth_element, inplace_merge and stable_partition.

What's missing from this patch is debug-iterator and container specializations
that are present for some of the STL-style algorithms that need to be ported
over to the ranges algos.  I marked them missing at TODO comments.  There are
also some other minor outstanding TODOs.

The code that could use the most thorough review is ranges::__copy_or_move,
ranges::__copy_or_move_backward, ranges::__equal and
ranges::__lexicographical_compare.  In the tests, I tried to test the interface
of each new overload, as well as the correctness of the new implementation.

libstdc++-v3/ChangeLog:

	Implement C++20 constrained algorithms
	* include/Makefile.am: Add new header.
	* include/Makefile.in: Regenerate.
	* include/std/algorithm: Include <bits/ranges_algo.h>.
	* include/bits/ranges_algo.h: New file.
	* testsuite/25_algorithms/adjacent_find/constrained.cc: New test.
	* testsuite/25_algorithms/all_of/constrained.cc: New test.
	* testsuite/25_algorithms/any_of/constrained.cc: New test.
	* testsuite/25_algorithms/binary_search/constrained.cc: New test.
	* testsuite/25_algorithms/copy/constrained.cc: New test.
	* testsuite/25_algorithms/copy_backward/constrained.cc: New test.
	* testsuite/25_algorithms/copy_if/constrained.cc: New test.
	* testsuite/25_algorithms/copy_n/constrained.cc: New test.
	* testsuite/25_algorithms/count/constrained.cc: New test.
	* testsuite/25_algorithms/count_if/constrained.cc: New test.
	* testsuite/25_algorithms/equal/constrained.cc: New test.
	* testsuite/25_algorithms/equal_range/constrained.cc: New test.
	* testsuite/25_algorithms/fill/constrained.cc: New test.
	* testsuite/25_algorithms/fill_n/constrained.cc: New test.
	* testsuite/25_algorithms/find/constrained.cc: New test.
	* testsuite/25_algorithms/find_end/constrained.cc: New test.
	* testsuite/25_algorithms/find_first_of/constrained.cc: New test.
	* testsuite/25_algorithms/find_if/constrained.cc: New test.
	* testsuite/25_algorithms/find_if_not/constrained.cc: New test.
	* testsuite/25_algorithms/for_each/constrained.cc: New test.
	* testsuite/25_algorithms/generate/constrained.cc: New test.
	* testsuite/25_algorithms/generate_n/constrained.cc: New test.
	* testsuite/25_algorithms/heap/constrained.cc: New test.
	* testsuite/25_algorithms/includes/constrained.cc: New test.
	* testsuite/25_algorithms/inplace_merge/constrained.cc: New test.
	* testsuite/25_algorithms/is_partitioned/constrained.cc: New test.
	* testsuite/25_algorithms/is_permutation/constrained.cc: New test.
	* testsuite/25_algorithms/is_sorted/constrained.cc: New test.
	* testsuite/25_algorithms/is_sorted_until/constrained.cc: New test.
	* testsuite/25_algorithms/lexicographical_compare/constrained.cc: New
	test.
	* testsuite/25_algorithms/lower_bound/constrained.cc: New test.
	* testsuite/25_algorithms/max/constrained.cc: New test.
	* testsuite/25_algorithms/max_element/constrained.cc: New test.
	* testsuite/25_algorithms/merge/constrained.cc: New test.
	* testsuite/25_algorithms/min/constrained.cc: New test.
	* testsuite/25_algorithms/min_element/constrained.cc: New test.
	* testsuite/25_algorithms/minmax/constrained.cc: New test.
	* testsuite/25_algorithms/minmax_element/constrained.cc: New test.
	* testsuite/25_algorithms/mismatch/constrained.cc: New test.
	* testsuite/25_algorithms/move/constrained.cc: New test.
	* testsuite/25_algorithms/move_backward/constrained.cc: New test.
	* testsuite/25_algorithms/next_permutation/constrained.cc: New test.
	* testsuite/25_algorithms/none_of/constrained.cc: New test.
	* testsuite/25_algorithms/nth_element/constrained.cc: New test.
	* testsuite/25_algorithms/partial_sort/constrained.cc: New test.
	* testsuite/25_algorithms/partial_sort_copy/constrained.cc: New test.
	* testsuite/25_algorithms/partition/constrained.cc: New test.
	* testsuite/25_algorithms/partition_copy/constrained.cc: New test.
	* testsuite/25_algorithms/partition_point/constrained.cc: New test.
	* testsuite/25_algorithms/prev_permutation/constrained.cc: New test.
	* testsuite/25_algorithms/remove/constrained.cc: New test.
	* testsuite/25_algorithms/remove_copy/constrained.cc: New test.
	* testsuite/25_algorithms/remove_copy_if/constrained.cc: New test.
	* testsuite/25_algorithms/remove_if/constrained.cc: New test.
	* testsuite/25_algorithms/replace/constrained.cc: New test.
	* testsuite/25_algorithms/replace_copy/constrained.cc: New test.
	* testsuite/25_algorithms/replace_copy_if/constrained.cc: New test.
	* testsuite/25_algorithms/replace_if/constrained.cc: New test.
	* testsuite/25_algorithms/reverse/constrained.cc: New test.
	* testsuite/25_algorithms/reverse_copy/constrained.cc: New test.
	* testsuite/25_algorithms/rotate/constrained.cc: New test.
	* testsuite/25_algorithms/rotate_copy/constrained.cc: New test.
	* testsuite/25_algorithms/search/constrained.cc: New test.
	* testsuite/25_algorithms/search_n/constrained.cc: New test.
	* testsuite/25_algorithms/set_difference/constrained.cc: New test.
	* testsuite/25_algorithms/set_intersection/constrained.cc: New test.
	* testsuite/25_algorithms/set_symmetric_difference/constrained.cc: New
	test.
	* testsuite/25_algorithms/set_union/constrained.cc: New test.
	* testsuite/25_algorithms/shuffle/constrained.cc: New test.
	* testsuite/25_algorithms/sort/constrained.cc: New test.
	* testsuite/25_algorithms/stable_partition/constrained.cc: New test.
	* testsuite/25_algorithms/stable_sort/constrained.cc: New test.
	* testsuite/25_algorithms/swap_ranges/constrained.cc: New test.
	* testsuite/25_algorithms/transform/constrained.cc: New test.
	* testsuite/25_algorithms/unique/constrained.cc: New test.
	* testsuite/25_algorithms/unique_copy/constrained.cc: New test.
	* testsuite/25_algorithms/upper_bound/constrained.cc: New test.