Separate space labeling from propagator trait (`deep_clone`)

[ptal]

Due to differents issues, the deep_clone method is inside the Propagator trait, it'd be wise to use a separate trait:

pub trait DeepClonePropagator<V>
{
  fn deep_clone(&self, cloned_vars: &Vec<V>) -> Self;
}

[ptal]

Trait erasure is not currently working in Rust. See rust-lang/rust#21814