Implement new serial algorithms from Parallelism TS (P0024R2)

Implement new serial algorithms from Parallelism TS (P0024R2)

These new (non-parallel) algorithms were added to C++17 along with the
parallel algorithms, but were missing from libstdc++.

	* include/bits/algorithmfwd.h: Change title of doc group.
	* include/bits/stl_algo.h (for_each_n): Add new C++17 algorithm from
	P0024R2.
	* include/bits/stl_numeric.h: Define doc group and add algos to it.
	* include/std/numeric (__is_random_access_iter): New internal trait.
	(reduce, transform_reduce, exclusive_scan, inclusive_scan)
	(transform_exclusive_scan, transform_inclusive_scan): Likewise.
	* testsuite/25_algorithms/for_each/for_each_n.cc: New test.
	* testsuite/26_numerics/exclusive_scan/1.cc: New test.
	* testsuite/26_numerics/inclusive_scan/1.cc: New test.
	* testsuite/26_numerics/reduce/1.cc: New test.
	* testsuite/26_numerics/transform_exclusive_scan/1.cc: New test.
	* testsuite/26_numerics/transform_inclusive_scan/1.cc: New test.
	* testsuite/26_numerics/transform_reduce/1.cc: New test.
	* testsuite/util/testsuite_iterators.h (test_container::size()): New
	member function.

From-SVN: r272459