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

src/bytes.rs

@@ -548,8 +548,14 @@ impl Buf for Bytes {
         }
     }
 
-    fn to_bytes(&mut self) -> crate::Bytes {
-        core::mem::replace(self, Bytes::new())
+    fn copy_to_bytes(&mut self, len: usize) -> crate::Bytes {
+        if len == self.remaining() {
+            core::mem::replace(self, Bytes::new())
+        } else {
+            let ret = self.slice(..len);
+            self.advance(len);
+            ret
+        }
     }
 }