[chore] clippy updates - use Entry::or_default (#592)

rust 1.73 is now stable and main breaks on these three. `clippy --fix`
did the necessary :)

src/algorithm/nest_cfgs.rs

@@ -345,10 +345,7 @@ impl<T: Copy + Clone + PartialEq + Eq + Hash> EdgeClassifier<T> {
             if min1dfs < n_dfs {
                 bs.push(Bracket::Capping(min1dfs, n));
                 // mark capping edge to be removed when we return out to the other end
-                self.capping_edges
-                    .entry(min1dfs)
-                    .or_insert(Vec::new())
-                    .push(n);
+                self.capping_edges.entry(min1dfs).or_default().push(n);
             }
         }
 
@@ -368,7 +365,7 @@ impl<T: Copy + Clone + PartialEq + Eq + Hash> EdgeClassifier<T> {
             self.edge_classes.entry(e).or_insert_with(|| Some((b, 0)));
         }
         // And capping backedges
-        for src in self.capping_edges.remove(&n_dfs).unwrap_or(Vec::new()) {
+        for src in self.capping_edges.remove(&n_dfs).unwrap_or_default() {
             bs.delete(&Bracket::Capping(n_dfs, src), &mut self.deleted_backedges)
         }
 

src/extension/infer.rs

@@ -201,10 +201,7 @@ impl UnificationContext {
 
     /// Declare a constraint on the metavariable
     fn add_constraint(&mut self, m: Meta, c: Constraint) {
-        self.constraints
-            .entry(m)
-            .or_insert(HashSet::new())
-            .insert(c);
+        self.constraints.entry(m).or_default().insert(c);
     }
 
     /// Declare that a meta has been solved