Use unwrap_or

Currently we are using `map_or` in a convoluted manner, we can just use
`unwrap_or` to get the inner value or default to 0.

Refactor only, no logic changes.

Author: tcharding

src/policy/semantic.rs

@@ -414,7 +414,7 @@ impl<Pk: MiniscriptKey> Policy<Pk> {
                     .count();
 
                 let n = subs.len() - unsatisfied_count - trivial_count; // remove all true/false
-                let m = k.checked_sub(trivial_count).map_or(0, |x| x); // satisfy all trivial
+                let m = k.checked_sub(trivial_count).unwrap_or(0); // satisfy all trivial
 
                 // m == n denotes `and` and m == 1 denotes `or`
                 let is_and = m == n;