Consider introducing trait to remove a lot of `Clone + Send + Sync` boilerplate

[GregoryConrad]

We could either rename CapsuleData to something else to add this new trait (naming it CapsuleData), or we could just add in the new trait as so:

pub trait CData: Clone + Send + Sync {}
impl<T: Clone + Send + Sync> CData for T {}

This will help implementation logic some here and there, but most importantly, this will help library users when creating capsules that are higher order functions. Example:

// Before:
fn stateful(
    CapsuleHandle { register, .. }: CapsuleHandle,
) -> (u32, impl Fn(u32) + Clone + Send + Sync) {
    let (state, set_state) = register.register(side_effects::state(0));
    (*state, set_state)
}

// After:
fn stateful(CapsuleHandle { register, .. }: CapsuleHandle) -> (u32, impl CData + Fn(u32)) {
    let (state, set_state) = register.register(side_effects::state(0));
    (*state, set_state)
}

Not sure on what the name should be. Shorter is better because this will be abundant in user code, but we also don't want to lose meaning.

[GregoryConrad]

Also, this would have the added benefit of other container implementations (different crates) being able to expose the exact same (give or take) API while requiring different traits to be in a container (like perhaps a single threaded container wouldn't need Send + Sync, so it's CData is only impl<T: Clone> CData for T ).

[GregoryConrad]

Fixed in c4b36fb8