Merge #131

131: Better subscribe API r=Woyten a=Woyten

This PR improves the usability of the `subscribe` API. The current API has the following problems:

- It is impossible to implement `SubscribableCallback` for different `FnMut` traits. This is the reason why the odd `WithCallback` helper objects were introduced.
- `SubscribableCallback` is crate private by accident. As a result, it is hard to reason what types actually implement `SubscribableCallback`

Besides that, the following APIs have been made more consistent:
- LEDs
- Buttons
- GPIO
- Temperature

Co-authored-by: Woyten <woyten.tielesch@online.de>