specialize io::copy to use copy_file_range, splice or sendfile

Currently it only applies to linux systems. It can be extended to make use
of similar syscalls on other unix systems.

Author: the8472

library/std/src/fs.rs

@@ -1656,7 +1656,7 @@ pub fn rename<P: AsRef<Path>, Q: AsRef<Path>>(from: P, to: Q) -> io::Result<()>
 /// the length of the `to` file as reported by `metadata`.
 ///
 /// If you’re wanting to copy the contents of one file to another and you’re
-/// working with [`File`]s, see the [`io::copy`] function.
+/// working with [`File`]s, see the [`io::copy()`] function.
 ///
 /// # Platform-specific behavior
 ///