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/parser/src/shortcuts.rs

@@ -80,8 +80,8 @@ impl<'a> LexedStr<'a> {
             State::PendingEnter | State::Normal => unreachable!(),
         }
 
-        let is_eof = builder.pos == builder.lexed.len();
-        is_eof
+        // is_eof?
+        builder.pos == builder.lexed.len()
     }
 }