Optimize Bytes::slice(n, n) (#123)

Return empty `Bytes` object

Bench for `slice_empty` difference is

```
55 ns/iter (+/- 1) # before this patch
17 ns/iter (+/- 5) # with this patch
```

Bench for `slice_not_empty` is

```
25,058 ns/iter (+/- 1,099) # before this patch
25,072 ns/iter (+/- 1,593) # with this patch
```

benches/bytes.rs

@@ -4,7 +4,7 @@ extern crate bytes;
 extern crate test;
 
 use test::Bencher;
-use bytes::{BytesMut, BufMut};
+use bytes::{Bytes, BytesMut, BufMut};
 
 #[bench]
 fn alloc_small(b: &mut Bencher) {
@@ -126,3 +126,23 @@ fn drain_write_drain(b: &mut Bencher) {
         test::black_box(parts);
     })
 }
+
+#[bench]
+fn slice_empty(b: &mut Bencher) {
+    b.iter(|| {
+        // Use empty vec to avoid measure of allocation/deallocation
+        let bytes = Bytes::from(Vec::new());
+        (bytes.slice(0, 0), bytes)
+    })
+}
+
+#[bench]
+fn slice_not_empty(b: &mut Bencher) {
+    b.iter(|| {
+        let b = Bytes::from(b"aabbccddeeffgghh".to_vec());
+        for _ in 0..1024 {
+            test::black_box(b.slice(3, 5));
+            test::black_box(&b);
+        }
+    })
+}