Add support for mapping UUIDs to/from byte arrays

[sophokles73]

The UUID struct currently only supports mapping to/from its hyphenated string representation. However, the fact that the UUID basically is just 16 consecutive bytes, seems to suggest that it should be possible, e.g. for transport implementations, to map it to a byte array in order to reduce the size of PDUs.

[PLeVasseur]

Makes sense to me as a feature.