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
5ce55f3ff5