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

examples/file_dialog/src/main.rs

@@ -64,7 +64,8 @@ impl eframe::App for MyApp {
                             additional_info.push(format!("{} bytes", bytes.len()));
                         }
                         if !additional_info.is_empty() {
-                            info += &format!(" ({})", additional_info.join(", "));
+                            use std::fmt::Write as _;
+                            write!(info, " ({})", additional_info.join(", ")).ok();
                         }
 
                         ui.label(info);