Enable a few more clippy lints (#8064)

Enable these new clippy lints and fix all warnings:

* `format_push_string` — use `write!` instead of `s += &format!(…)` to
avoid extra allocations
* `ignored_unit_patterns` — use `()` instead of `_` when matching unit
* `missing_fields_in_debug` — ensure manual `Debug` impls account for
all fields
* `needless_raw_string_hashes` — remove unnecessary `r#` on string
literals
* `ref_option` — prefer `Option<&T>` over `&Option<T>` in function
signatures

crates/eframe/src/native/file_storage.rs

@@ -207,7 +207,7 @@ fn save_to_disk(file_path: &PathBuf, kv: &HashMap<String, String>) {
             profiling::scope!("ron::serialize");
             if let Err(err) = ron::Options::default()
                 .to_io_writer_pretty(&mut writer, &kv, config)
-                .and_then(|_| writer.flush().map_err(|err| err.into()))
+                .and_then(|()| writer.flush().map_err(|err| err.into()))
             {
                 log::warn!("Failed to serialize app state: {err}");
             } else {