BufMut::put_bytes(self, val, cnt) (#487)

Equivalent to

```
for _ in 0..cnt {
    self.put_u8(val);
}
```

but may work faster.

Name and signature is chosen to be consistent with `ptr::write_bytes`.

Include three specializations:
* `Vec<u8>`
* `&mut [u8]`
* `BytesMut`

`BytesMut` and `&mut [u8]` specializations use `ptr::write`, `Vec<u8>`
specialization uses `Vec::resize`.

tests/test_bytes.rs

@@ -986,3 +986,11 @@ fn bytes_with_capacity_but_empty() {
     let vec = Vec::with_capacity(1);
     let _ = Bytes::from(vec);
 }
+
+#[test]
+fn bytes_put_bytes() {
+    let mut bytes = BytesMut::new();
+    bytes.put_u8(17);
+    bytes.put_bytes(19, 2);
+    assert_eq!([17, 19, 19], bytes.as_ref());
+}