perf: fix O(n²) string allocations in fold operations

vm/src/vm/errors/vm_exception.rs

@@ -154,11 +154,13 @@ fn substitute_error_message_references(
         }
         if !invalid_references.is_empty() {
             // Add the invalid references (if any) to the error_msg
+            let references = invalid_references
+                .iter()
+                .map(|arg| format!("'{arg}'"))
+                .collect::<Vec<_>>()
+                .join(", ");
             error_msg.push_str(&format!(
-                " (Cannot evaluate ap-based or complex references: [{}])",
-                invalid_references
-                    .iter()
-                    .fold(String::new(), |acc, arg| acc + &format!("'{arg}'"))
+                " (Cannot evaluate ap-based or complex references: [{references}])"
             ));
         }
     }

vm/src/vm/runners/cairo_pie.rs

@@ -619,9 +619,14 @@ pub(super) mod serde_impl {
             };
         }
 
-        let string = res
-            .iter()
-            .fold(String::new(), |string, b| string + &format!("{:02x}", b));
+        let string = res.iter().fold(
+            String::with_capacity(res.len() * 2),
+            |mut string, b| {
+                use core::fmt::Write;
+                write!(&mut string, "{:02x}", b).unwrap();
+                string
+            },
+        );
 
         serializer.serialize_str(&string)
     }