* include/std/future (promise): Add constructors for uses-allocator
	construction from rvalue promise.
	(packaged_task): Implement LWG 2067. Add additional constructors for
	uses-allocator construction.
	* testsuite/30_threads/packaged_task/cons/3.cc: New.
	* testsuite/30_threads/packaged_task/cons/alloc2.cc: New.
	* testsuite/30_threads/promise/cons/alloc2.cc: New.

From-SVN: r180757