[feature] automatically find `iterator_system` from system of associated pointer type

I think it would be very useful if, for iterator-like classes, `thrust::iterator_system` resolves automatically (by default) to `thrust::iterator_system::pointer>`.

This way the execution policy of an algorithm would result in the same execution policy as the underlying memory.

Currently I have to do something like:

```
namespace thrust{
template
struct iterator_system>{
using type = typename thrust::iterator_system>::pointer>::type;
};
}
```

And I can't do better because, currently, the primary template of `thrust::iterator_system` is not SFINAE friendly (no `, void = Dummy>` to exploit).