Extract app trait into apdu-app

[robin-nitrokey]

Currently, all APDU applications depend on apdu-dispatch to implement the App trait. This means that they also need to be updated if the dispatch implementation is changed. This causes unnecessary releases and maintenance work. By introducing the apdu-app crate, we can change the dispatch implementation without affecting the applications.

[sosthene-nitrokey]

Before merging it we could also update the App trait:

Replace Command<C> with CommandView (one less const generic.
Bump heapless to 0.8. Sadly heapless hasn't yet released the VecView feature so we still have one const generic, but bumping the heapless version is still better. Since we have released heapless-bytes, we can start migrating everything to heapless 0.8.

[robin-nitrokey]

For crates like this one it would be good to have a trait that provides the Bytes/Vec semantics (push, extend_from_slice) without depending on a specific implementation.

[robin-nitrokey]

Replacing Command with CommandView is a bigger change. Let’s do it in a separate PR. Regarding heapless, I’m no longer sure if it is really worth the effort to migrate now. What is the benefit without the view types? We will need another migration once 0.9 is released anyway.