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
```

tests/test_bytes.rs

@@ -79,6 +79,15 @@ fn slice() {
     let b = a.slice(3, 5);
     assert_eq!(b, b"lo"[..]);
 
+    let b = a.slice(0, 0);
+    assert_eq!(b, b""[..]);
+
+    let b = a.slice(3, 3);
+    assert_eq!(b, b""[..]);
+
+    let b = a.slice(a.len(), a.len());
+    assert_eq!(b, b""[..]);
+
     let b = a.slice_to(5);
     assert_eq!(b, b"hello"[..]);