expose better `witness_vars` in backend

[mimoo]

currently the backend exposes:

fn witness_vars(&self) -> &HashMap<usize, Value<Self>>;

I think we should expose instead:

fn witness_vars(&self, var: &CellVar) -> Value<Self>;

and rename it to get_cellvar_value

[Teja-11]

can i work on this?

[mimoo]

Sounds good :)