Update the benchmarks to use rand_chacha directly

benches/me.rs

@@ -11,7 +11,8 @@ use criterion::*;
 use crate::partition::*;
 use crate::partition::BlockSize::*;
 use crate::plane::*;
-use rand::{ChaChaRng, Rng, SeedableRng};
+use rand::{Rng, SeedableRng};
+use rand_chacha::ChaChaRng;
 use rav1e::me;
 use rav1e::Pixel;
 

benches/predict.rs

@@ -8,7 +8,8 @@
 // PATENTS file, you can obtain it at www.aomedia.org/license/patent.
 
 use criterion::*;
-use rand::{ChaChaRng, Rng, RngCore, SeedableRng};
+use rand::{Rng, RngCore, SeedableRng};
+use rand_chacha::ChaChaRng;
 use rav1e::partition::BlockSize;
 use rav1e::predict::{Block4x4, Intra};
 use crate::plane::*;

benches/transform.rs

@@ -8,7 +8,8 @@
 // PATENTS file, you can obtain it at www.aomedia.org/license/patent.
 
 use criterion::*;
-use rand::{ChaChaRng, Rng, SeedableRng};
+use rand::{Rng, SeedableRng};
+use rand_chacha::ChaChaRng;
 use rav1e::transform;
 
 fn bench_idct4(b: &mut Bencher, bit_depth: &usize) {