Bytes::extend_from_slice (#120)

`extend_with_slice` is super-convenient operation on `Bytes`.

While `put_u8` would be expensive on `Bytes`, `extend_from_slice`
is OK, because it is batch, and it checks for kind only once.

Patch also adds `impl Extend for Bytes`.

cc #116

tests/test_bytes.rs

@@ -357,12 +357,39 @@ fn inline_storage() {
 }
 
 #[test]
-fn extend() {
+fn extend_mut() {
     let mut bytes = BytesMut::with_capacity(0);
     bytes.extend(LONG);
     assert_eq!(*bytes, LONG[..]);
 }
 
+#[test]
+fn extend_shr() {
+    let mut bytes = Bytes::new();
+    bytes.extend(LONG);
+    assert_eq!(*bytes, LONG[..]);
+}
+
+#[test]
+fn extend_from_slice_mut() {
+    for &i in &[3, 34] {
+        let mut bytes = BytesMut::new();
+        bytes.extend_from_slice(&LONG[..i]);
+        bytes.extend_from_slice(&LONG[i..]);
+        assert_eq!(LONG[..], *bytes);
+    }
+}
+
+#[test]
+fn extend_from_slice_shr() {
+    for &i in &[3, 34] {
+        let mut bytes = Bytes::new();
+        bytes.extend_from_slice(&LONG[..i]);
+        bytes.extend_from_slice(&LONG[i..]);
+        assert_eq!(LONG[..], *bytes);
+    }
+}
+
 #[test]
 fn from_static() {
     let mut a = Bytes::from_static(b"ab");