Add Buf::copy_to_bytes(len) (#439)

This method replaces `Buf::to_bytes()`, providing a method that copies a
subset of the remaining buffer into a `Bytes` value. As this is strictly
more flexible, `to_bytes()` is removed.

Fixes: #129, #398

tests/test_chain.rs

@@ -9,7 +9,7 @@ fn collect_two_bufs() {
     let a = Bytes::from(&b"hello"[..]);
     let b = Bytes::from(&b"world"[..]);
 
-    let res = a.chain(b).to_bytes();
+    let res = a.chain(b).copy_to_bytes(10);
     assert_eq!(res, &b"helloworld"[..]);
 }