Simplify iteration idioms (#13834)

Remove unnecessary uses of `.as_ref()`, `.iter()`, `&**` and similar, mostly in situations when iterating over variables. Many of these changes are only possible following #13826, when we bumped our MSRV to 1.80: several useful implementations on `&Box<[T]>` were only stabilised in Rust 1.80. Some of these changes we could have done earlier, however.

crates/red_knot_python_semantic/src/semantic_index/builder.rs

@@ -1012,7 +1012,7 @@ where
 
                 // Add symbols and definitions for the parameters to the lambda scope.
                 if let Some(parameters) = &lambda.parameters {
-                    for parameter in &**parameters {
+                    for parameter in parameters {
                         self.declare_parameter(parameter);
                     }
                 }