[Constraint] Introduce ConstraintSystemReference idea for friendly API

[alxiong]

From @zhenfeizhang in #94 (comment)

In general I want to advocate my "circuit ref" idea. For example

struct BoolVar<'a> {
  index: Variable,
  cs: &'a ConstraintSystem
}

On top of overloading operators here are some additional thoughts:

• to access the variable, we should use an API get_index(&self)->Variable -- i think it is a bit cleaner than into()
  • it makes more sense to me to implement Into/From for Vec<BoolVar> and Variable, in both directions, which are binary (de)composition
• to create new variables we should use associated function BoolVar::new(cs: &ConstraintSystem, value: bool) rather than Circuit::create_bool_variable(...) and so on...
• to check variable bound we can simply call bool_var.check_bound(), and we do not need to pass parameters

• add CSRef for BoolVar
• add CSRef for PointVar
• add CSRef for Variable