impl Default, Copy, Clone for std::io::Sink and Empty

The omission of Sink: Default is causing me a slight inconvenience in
a test harness.  There seems little reason for this and Empty not to
be Clone and Copy too.

I have made all three of these insta-stable, because:

AIUI Copycan only be derived, and I was not able to find any
examples of how to unstably derive it.  I think it is probably not
possible.

I hunted through the git history for precedent and found

  79b8ad8
  Implement `Copy` for `IoSlice`
  rust-lang#69403

which was also insta-stable.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>

library/std/src/io/util.rs

@@ -14,6 +14,7 @@ use crate::io::{
 /// the documentation of [`empty()`] for more details.
 #[stable(feature = "rust1", since = "1.0.0")]
 #[non_exhaustive]
+#[derive(Copy, Clone, Default)]
 pub struct Empty;
 
 /// Constructs a new handle to an empty reader.
@@ -172,6 +173,7 @@ impl fmt::Debug for Repeat {
 /// see the documentation of [`sink()`] for more details.
 #[stable(feature = "rust1", since = "1.0.0")]
 #[non_exhaustive]
+#[derive(Copy, Clone, Default)]
 pub struct Sink;
 
 /// Creates an instance of a writer which will successfully consume all data.