Add feature to support platforms without atomic CAS (#467)

src/buf/chain.rs

@@ -1,5 +1,5 @@
 use crate::buf::{IntoIter, UninitSlice};
-use crate::{Buf, BufMut, Bytes};
+use crate::{Buf, BufMut};
 
 #[cfg(feature = "std")]
 use std::io::IoSlice;
@@ -169,7 +169,7 @@ where
         n
     }
 
-    fn copy_to_bytes(&mut self, len: usize) -> Bytes {
+    fn copy_to_bytes(&mut self, len: usize) -> crate::Bytes {
         let a_rem = self.a.remaining();
         if a_rem >= len {
             self.a.copy_to_bytes(len)