Normalizing ExtMap, Stamps, and others to remove empty rows

[samlown]

• ExtMap values are now normalized, removing any empty values.
• Stamps also normalized
• UUID in org.Party is now cleaning UUID.

[samlown]

I still find surprising ExtMap is under tax and not under a more generic package such us cbc, but I believe we already had and settled that discussion.

I think the main confusion there is with the values, I'm not happy with the KeyOrCode type. I'm tempted to convert it into a string without strict validation: most extensions have a fixed list of constants anyway.

I'm also tempted to rename into simply Extensions instead of ExtMap. That way the implications of tax.Extensions is way more obvious.