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/syntax/src/ptr.rs

@@ -82,7 +82,7 @@ impl<N: AstNode> AstPtr<N> {
 
     /// Like `SyntaxNodePtr::cast` but the trait bounds work out.
     pub fn try_from_raw(raw: SyntaxNodePtr) -> Option<AstPtr<N>> {
-        N::can_cast(raw.kind()).then(|| AstPtr { raw, _ty: PhantomData })
+        N::can_cast(raw.kind()).then_some(AstPtr { raw, _ty: PhantomData })
     }
 }