Extend thrust::detail::allocator_traits to allow access to an allocator's system

[jaredhoberock]

Consider adding allocator_traits<Alloc>::system() which would simply return a default-constructed iterator_system<Ptr>::type.

Allocators with a system reference inside could return a reference to it.

This would help localize the instances of default-constructed systems.

[jaredhoberock]

This trait should check the allocator for the member function system(), and use that if it exists instead of having to specialize for each different allocator which might want to include this function.