20250209 bounded static var refs

[shnarazk]

cf. #283

I have to re-consider the efficiency by this approach. It should reduce indirect deferences I thought. But in fact, this gets worse results.

• each literal has one reference to Var and one boolean constraint. So this needs more space. It's not good in terms of reference locality. On the other hand, the current scheme uses an int that encodes VarId and a boolean constraint.
• There's no or little difference between getting a direct reference to a Var and holding an offset for Vec<Var> at CPU instruction level.