Mark BytesMut::extend_from_slice as inline (#595)

This function can be hot in applications that do a lot of encoding. Ideally would do the same for `<BytesMut as BufMut>::put_slice` and `<BytesMut as BufMut::put_u8`.

src/bytes_mut.rs

@@ -761,6 +761,7 @@ impl BytesMut {
     ///
     /// assert_eq!(b"aaabbbcccddd", &buf[..]);
     /// ```
+    #[inline]
     pub fn extend_from_slice(&mut self, extend: &[u8]) {
         let cnt = extend.len();
         self.reserve(cnt);