LWG 2921 remove packaged_task constructors taking allocators

	* doc/xml/manual/evolution.xml: Document LWG DR 2921 change.
	* doc/xml/manual/intro.xml: Likewise.
	* include/std/future (__create_task_state): Add default arguments
	to make providing an allocator optional.
	(packaged_task::packaged_task(F&&)): Call __create_task_state directly
	instead of delegating to another constructor.
	(packaged_task::packaged_task(allocator_arg_t, const A&, ...)): Do not
	define allocator-extended constructors for C++17 and later.
	* testsuite/30_threads/packaged_task/cons/alloc.cc: Only run test for
	C++11 and C++14.
	* testsuite/30_threads/packaged_task/cons/alloc2.cc: Likewise.
	* testsuite/30_threads/packaged_task/cons/alloc_min.cc: Likewise.
	* testsuite/30_threads/packaged_task/uses_allocator.cc: Likewise.

From-SVN: r271582