Moar linting: needless_borrow, let_unit_value, ...

* There are a few needless borrows that don't seem to be needed. I even did a quick assembly comparison and posted a q to stackoveflow on it. See [here](https://stackoverflow.com/questions/74910196/advantages-of-pass-by-ref-val-with-impl-intoiteratoritem-impl-asrefstr)
* removed several `let _ = ...` when they don't look necessary (even a few ones that were not suggested by clippy (?))
* there were a few `then(|| ctor{})` that clippy suggested to replace with `then_some(ctor{})` -- seems reasonable?
* some unneeded assignment+return - keep the code a bit leaner
* a few `writeln!` instead of `write!`, or even consolidate write!
* a nice optimization to use `ch.is_ascii_digit` instead of `ch.is_digit(10)`

crates/test-utils/src/lib.rs

@@ -396,7 +396,7 @@ pub fn skip_slow_tests() -> bool {
         eprintln!("ignoring slow test");
     } else {
         let path = project_root().join("./target/.slow_tests_cookie");
-        fs::write(&path, ".").unwrap();
+        fs::write(path, ".").unwrap();
     }
     should_skip
 }