Add more predicate functions for testing RDF concepts

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)