fix: reduce memory healed by deferred objects

`bytes.Buffer` will always overallocate (for performance) and we don't
have a great way of predicting the correct size. So, instead, copy once
when we're done.

deferred.go

@@ -24,10 +24,7 @@ func (d *Deferred) MarshalCBOR(w io.Writer) error {
 }
 
 func (d *Deferred) UnmarshalCBOR(br io.Reader) (err error) {
-	// Reuse any existing buffers.
-	reusedBuf := d.Raw[:0]
-	d.Raw = nil
-	buf := bytes.NewBuffer(reusedBuf)
+	buf := bytes.NewBuffer(nil)
 
 	// Allocate some scratch space.
 	scratch := make([]byte, maxHeaderSize)
@@ -90,6 +87,7 @@ func (d *Deferred) UnmarshalCBOR(br io.Reader) (err error) {
 			return fmt.Errorf("unhandled deferred cbor type: %d", maj)
 		}
 	}
-	d.Raw = buf.Bytes()
+	d.Raw = d.Raw[:0]
+	d.Raw = append(d.Raw, buf.Bytes()...)
 	return nil
 }