Rollup merge of #111074 - WaffleLapkin:🌟unsizes_your_buf_reader🌟, r=Amanieu

Relax implicit `T: Sized` bounds on `BufReader<T>`, `BufWriter<T>` and `LineWriter<T>`

TL;DR:
```diff,rust
-pub struct BufReader<R> { /* ... */ }
+pub struct BufReader<R: ?Sized> { /* ... */ }

-pub struct BufWriter<W: Write> { /* ... */ }
+pub struct BufWriter<W: ?Sized + Write> { /* ... */ }

-pub struct LineWriter<W: Write> { /* ... */ }
+pub struct LineWriter<W: ?Sized + Write> { /* ... */ }
```

This allows using `&mut BufReader<dyn Read>`, for example.

**This is an insta-stable change**.

library/std/src/io/mod.rs

@@ -2754,7 +2754,7 @@ trait SizeHint {
     }
 }
 
-impl<T> SizeHint for T {
+impl<T: ?Sized> SizeHint for T {
     #[inline]
     default fn lower_bound(&self) -> usize {
         0