More builtin types?

[johnstonskj]

Given the focus of SDML do we need more builtin types, or maybe more "prelude" types.

There are some candidates from XSD:

• duration
• date
• time
• dateTime

Also some that are related:

• binary -- which could be xsd:base64Binary or xsd:hexBinary
• dateInterval -- a start/end date
• timeInterval
• dateTimeInterval

Some that aren't covered:

• currency -- a decimal with specific rounding rules
• currencyCode -- ISO
• countryCode -- ISO
• languageCode -- ISO
• languageTag -- BCP