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.

library/std/src/lib.rs

@@ -316,6 +316,7 @@
 #![feature(toowned_clone_into)]
 #![feature(total_cmp)]
 #![feature(trace_macros)]
+#![feature(try_blocks)]
 #![feature(try_reserve)]
 #![feature(unboxed_closures)]
 #![feature(unsafe_block_in_unsafe_fn)]