Polymorphism

[matt-beanland]

The initial version of ash_outstanding expects the actual struct name to match exactly. This is probably not going to work for all use cases, since it is reasonable that you might require polymorphism, where a generic expectation is realised by resources which don't have the same module (struct) name.

One design would be to simply provide an option to expect the struct name to match or not, in which case we simply allow expected to be any struct or even a map, and outstanding would not be a struct, rather would just be a map.