expands weighted-shuffle benchmarks (anza-xyz#179)

Adding separate benchmarks for WeightedShuffle::new and
WeightedShuffle::shuffle.

gossip/benches/weighted_shuffle.rs

@@ -11,18 +11,28 @@ use {
 };
 
 fn make_weights<R: Rng>(rng: &mut R) -> Vec<u64> {
-    repeat_with(|| rng.gen_range(1..100)).take(1000).collect()
+    repeat_with(|| rng.gen_range(1..100)).take(4000).collect()
 }
 
 #[bench]
-fn bench_weighted_shuffle(bencher: &mut Bencher) {
+fn bench_weighted_shuffle_new(bencher: &mut Bencher) {
+    let mut rng = rand::thread_rng();
+    bencher.iter(|| {
+        let weights = make_weights(&mut rng);
+        std::hint::black_box(WeightedShuffle::new("", &weights));
+    });
+}
+
+#[bench]
+fn bench_weighted_shuffle_shuffle(bencher: &mut Bencher) {
     let mut seed = [0u8; 32];
     let mut rng = rand::thread_rng();
     let weights = make_weights(&mut rng);
+    let weighted_shuffle = WeightedShuffle::new("", &weights);
     bencher.iter(|| {
         rng.fill(&mut seed[..]);
-        WeightedShuffle::new("", &weights)
-            .shuffle(&mut ChaChaRng::from_seed(seed))
-            .collect::<Vec<_>>()
+        let mut rng = ChaChaRng::from_seed(seed);
+        let shuffle = weighted_shuffle.clone().shuffle(&mut rng);
+        std::hint::black_box(shuffle.collect::<Vec<_>>());
     });
 }