Create `toStringBytes` method that returns a `byte[]` instead of a String

[l0rinc]

For even better performance, it could even take an existing byte[] array, fill it with values and not create any garbage. This would be great especially on Java 9+, where char[] aren't the internal representations of UUID Strings anymore. It would be cool if it would a get a byte array for source as input (see https://github.com/jchambers/fast-uuid/issues/9), maybe even with an offset as length, like System.arraycopy or Arrays.equals does.

[jchambers]

Can you say more about the use case for this feature? As with #9, I'm not necessarily opposed, but am trying to understand why you'd want to do that.