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/sys/unix/kernel_copy.rs

@@ -466,7 +466,7 @@ impl<T: CopyRead> CopyRead for Take<T> {
     }
 }
 
-impl<T: CopyRead> CopyRead for BufReader<T> {
+impl<T: ?Sized + CopyRead> CopyRead for BufReader<T> {
     fn drain_to<W: Write>(&mut self, writer: &mut W, outer_limit: u64) -> Result<u64> {
         let buf = self.buffer();
         let buf = &buf[0..min(buf.len(), outer_limit.try_into().unwrap_or(usize::MAX))];
@@ -495,7 +495,7 @@ impl<T: CopyRead> CopyRead for BufReader<T> {
     }
 }
 
-impl<T: CopyWrite> CopyWrite for BufWriter<T> {
+impl<T: ?Sized + CopyWrite> CopyWrite for BufWriter<T> {
     fn properties(&self) -> CopyParams {
         self.get_ref().properties()
     }