rename std::vec_ng -> std::vec

Closes #12771

src/libnum/rational.rs

@@ -16,7 +16,7 @@ use std::cmp;
 use std::fmt;
 use std::from_str::FromStr;
 use std::num::{Zero,One,ToStrRadix,FromStrRadix,Round};
-use std::vec_ng::Vec;
+use std::vec::Vec;
 use bigint::{BigInt, BigUint, Sign, Plus, Minus};
 
 /// Represents the ratio between 2 numbers.