Rollup merge of #78811 - a1phyr:const_io_structs, r=dtolnay

Make some std::io functions `const`

Tracking issue: #78812

Make the following functions `const`:
- `io::Cursor::new`
- `io::Cursor::get_ref`
- `io::Cursor::position`
- `io::empty`
- `io::repeat`
- `io::sink`

r? `````@dtolnay`````

library/std/src/lib.rs

@@ -242,6 +242,7 @@
 #![feature(const_fn_transmute)]
 #![feature(const_fn)]
 #![feature(const_fn_fn_ptr_basics)]
+#![feature(const_io_structs)]
 #![feature(const_ip)]
 #![feature(const_ipv6)]
 #![feature(const_raw_ptr_deref)]