Function compute_policy_diff 

pub fn compute_policy_diff(old: &Expr, new: &Expr) -> Vec<PolicyChange>

Compute the semantic diff between two compiled policy expressions.

Args:

• old: The previous policy expression.
• new: The updated policy expression.

Usage:

let changes = compute_policy_diff(&old_expr, &new_expr);
let risk = overall_risk_score(&changes);