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/proc-macro-api/src/process.rs

@@ -67,7 +67,7 @@ impl Process {
         args: impl IntoIterator<Item = impl AsRef<OsStr>>,
     ) -> io::Result<Process> {
         let args: Vec<OsString> = args.into_iter().map(|s| s.as_ref().into()).collect();
-        let child = JodChild(mk_child(&path, &args)?);
+        let child = JodChild(mk_child(&path, args)?);
         Ok(Process { child })
     }