Make some std::io functions const

Includes:
- io::Cursor::new
- io::Cursor::get_ref
- io::Cursor::position
- io::empty
- io::repeat
- io::sink

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)]