Fix use of rand in liballoc benches

Author: udoprog

src/liballoc/benches/btree/map.rs

@@ -12,7 +12,7 @@
 use std::iter::Iterator;
 use std::vec::Vec;
 use std::collections::BTreeMap;
-use std::__rand::{Rng, thread_rng};
+use rand::{Rng, thread_rng};
 use test::{Bencher, black_box};
 
 macro_rules! map_insert_rand_bench {

src/liballoc/benches/slice.rs

@@ -8,7 +8,7 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-use std::__rand::{thread_rng};
+use rand::{thread_rng};
 use std::mem;
 use std::ptr;