rework initialization

[kaspar030]

This PR reworks (and hopefully simplifies) task initialization.

With SendCell and Delegate I think we got the tools to not use a trait (object) for initialization.
I'm pretty happy so far, it feels like much reduced boilerplate already.

For now the PR:

• drops the Application trait
• drops Drivers and InitializationArgs
• introduces SendCell, a struct that helps passing around non-Send types, runtime-checking that an object stays on the same executor
• introduces riot_rs::embassy::network_stack(), which is now used to get to the network stack Stack object instead of passing it through the Drivers struct
• we need a new macro for the tasks
• review & document SendCell. it's currently more of a PoC, we need to make sure it's sound, and polish the API
• introduce Delegate, which can be used to provide hooks e.g., for the UsbBuilder
• use Delegate to provide a hook into the UsbBuilder
• review & document Delegate

I'm aware that this is a pretty large PR as is. I'll split it up once we agree on the direction.

[kaspar030]

So here, b/c the Buttons peripherals are optional, I found it easier to just write a custom non-async init function.

[kaspar030]

Thanks for the review!
I've incorporated everyting and split the PR into #84, #85, #86, #87.