Add more predicate functions for testing RDF concepts

[RickMoynihan]

It'd be useful to have a protocol extended to all types to check for:

• uri?
• lang-string?
• xsd-string?
• resource? (URI or blank node)
• literal?
• non-native-literal? (a boxed literal, i.e. not one mapped to a native platform type)
• blank-node? (currently implemented as a multimethod)