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/egui_demo_app/src/main.rs

@@ -38,7 +38,8 @@ fn main() {
         });
         for loud_crate in ["naga", "wgpu_core", "wgpu_hal"] {
             if !rust_log.contains(&format!("{loud_crate}=")) {
-                rust_log += &format!(",{loud_crate}=warn");
+                use std::fmt::Write as _;
+                write!(rust_log, ",{loud_crate}=warn").ok();
             }
         }