Register buffers with an execution context.

template<
    typename Executor,
    typename MutableBufferSequence,
    typename Allocator>
buffer_registration< MutableBufferSequence, Allocator > register_buffers(
    const Executor & ex,
    const MutableBufferSequence & buffer_sequence,
    const Allocator & alloc,
    constraint_t< is_executor< Executor >::value||execution::is_executor< Executor >::value >  = 0);