internal/format: buffer newlineWriter writes

Most writes in the cmd/age Writer stack are chunk-sized, so
approximately 64KiB. However, the newlineWriter, which splits lines at
64 columns, was doing a Write on the underlying Writer for each line,
making chunks effectively 48 bytes (before base64). There is no
buffering underneath it, so it was resulting in a lot of write syscalls.

Add a reusable bytes.Buffer to buffer the output of each
(*newlineWriter).Write call, and Write it all at once on the
destination.

This makes --armor just 50% slower than plain, instead of 10x.

Fixes #167

cmd/age/age.go

@@ -154,7 +154,8 @@ func main() {
 		}
 	}
 
-	var in, out io.ReadWriter = os.Stdin, os.Stdout
+	var in io.Reader = os.Stdin
+	var out io.Writer = os.Stdout
 	if name := flag.Arg(0); name != "" && name != "-" {
 		f, err := os.Open(name)
 		if err != nil {