Snowcap: Add derive macro for Signal and Universal

[Ph4ntomas]

This PR adds derive macro for Signal and Universal.

The Signal macro add bounds Clone + 'static automatically on generic types. I did not implement any validation/pruning yet, meaning deriving signal for :

struct<Msg: Clone> AAA {
   [...]
}

Will lead to the following code being generated:

impl<Msg: Clone + Clone + 'static> Signal for AAA<Msg> {}

Which I don't quite like.

Draft until #430 is merged.

[Ottatop]

Thanks!