Hacking/cart extra

creating a PR so I can comment on ::Cart::Extra more easily
I'm confused about the use of HashRef rather than ArrayRef. Do we expect the key to be the same as the name attribute in ::Extra? This seems confusing to me. Maybe simpler to drop ::Extra and use a simple HashRef:
```
$product->add_extra( colour => { value => "red" }, length => 3.5, height => { value => 44, label => "Height" } )
```
Given this kind of setter we can add coercion so that all values end up containing { value => ..., label => ... }. Maybe we can add a new Interchange6::Type.