Add a benchmark for from_slice

Because it seems like good practice.

benches/bench.rs

@@ -63,6 +63,15 @@ fn bench_extend(b: &mut Bencher) {
     });
 }
 
+#[bench]
+fn bench_from_slice(b: &mut Bencher) {
+    let v: Vec<u64> = (0..100).collect();
+    b.iter(|| {
+        let vec: SmallVec<[u64; 16]> = SmallVec::from_slice(&v);
+        vec
+    });
+}
+
 #[bench]
 fn bench_extend_from_slice(b: &mut Bencher) {
     let v: Vec<u64> = (0..100).collect();