Specialize copy_to_bytes for Chain and Take (#481)

Avoid allocation when `Take` or `Chain` is composed of `Bytes`
objects.

This works now for `Take`.

`Chain` it works if the requested bytes does not cross boundary
between `Chain` members.

src/buf/take.rs

@@ -1,4 +1,4 @@
-use crate::Buf;
+use crate::{Buf, Bytes};
 
 use core::cmp;
 
@@ -144,4 +144,12 @@ impl<T: Buf> Buf for Take<T> {
         self.inner.advance(cnt);
         self.limit -= cnt;
     }
+
+    fn copy_to_bytes(&mut self, len: usize) -> Bytes {
+        assert!(len <= self.remaining(), "`len` greater than remaining");
+
+        let r = self.inner.copy_to_bytes(len);
+        self.limit -= len;
+        r
+    }
 }