channel: Use append to allocate an output buffer.

This improves the performance and decreases allocations for the split framings
appreciably.

 BENCHMARK                      BEFORE AFTER SPEEDUP B/op (B) B/op (A)
 BenchmarkFramingCost/Line-12   2547   2121  16.7%   4096     2048
 BenchmarkFramingCost/NUL-12    2564   2091  18.4%   4096     2048

channel/split.go

@@ -34,9 +34,7 @@ func (c split) Send(msg []byte) error {
 	if bytes.IndexByte(msg, c.split) >= 0 {
 		return errors.New("message contains split byte")
 	}
-	out := make([]byte, len(msg)+1)
-	copy(out, msg)
-	out[len(msg)] = c.split
+	out := append(msg, c.split)
 	_, err := c.wc.Write(out)
 	return err
 }