rename std::vec_ng -> std::vec

Closes #12771

src/test/compile-fail/issue-11873.rs

@@ -8,7 +8,7 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-use std::vec_ng::Vec;
+use std::vec::Vec;
 
 fn main() {
     let mut v = vec!(1);