Add experimental::sample and experimental::shuffle from N4531

Add experimental::sample and experimental::shuffle from N4531

The additions to <experimental/random> were added in 2015 but the new
algorithms in <experimental/algorithm> were not. This adds them.

	* include/experimental/algorithm (sample, shuffle): Add new overloads
	using per-thread random number engine.
	* testsuite/experimental/algorithm/sample.cc: Simpify and reduce
	dependencies by using __gnu_test::test_container.
	* testsuite/experimental/algorithm/sample-2.cc: New.
	* testsuite/experimental/algorithm/shuffle.cc: New.

From-SVN: r262024