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
```

src/bytes.rs

@@ -464,6 +464,11 @@ impl Bytes {
     /// Requires that `begin <= end` and `end <= self.len()`, otherwise slicing
     /// will panic.
     pub fn slice(&self, begin: usize, end: usize) -> Bytes {
+        if begin == end {
+            assert!(begin <= self.len());
+            return Bytes::new();
+        }
+
         let mut ret = self.clone();
 
         unsafe {