Simplify common case of use_future_t that uses std::allocator

Simplify common case of use_future_t that uses std::allocator

There is no need to store and pass around the allocator object when it's
an instance of std::allocator. Define a partial specialization of
std::use_future_t and the corresponding completion token so that no
allocator is stored. Overload the completion handler constructor to not
expect an allocator to be stored.

	* include/experimental/executor (__use_future_ct, use_future_t):
	Define partial specializations for std::allocator.
	(__use_future_ch): Overload constructor for completion tokens using
	std::allocator.

From-SVN: r277404