A recent change caused a performance regression. This restores the
previous performance and adds a performance test.

	* scripts/check_performance: Allow tests to choose a -std flag.
	* src/c++17/memory_resource.cc (bitset::get_first_unset()): Use local
	variables of the right types. Call update_next_word() unconditionally.
	* testsuite/20_util/unsynchronized_pool_resource/cons.cc: New test.
	* testsuite/performance/20_util/memory_resource/pools.cc: New test.
	* testsuite/util/testsuite_performance.h (time_counter): Allow
	timer to be restarted.

From-SVN: r266164