The turtle printer now:
- properly escapes IRIs;
- group the triples by subject, and then by predicate;
- can use a set of specified prefixes to shorten the IRIs;
- uses relative IRIs when they are relative to an internal base;
- can write down an explicit base;
- shorten integer, decimal, double and boolean literals;
- shorten the rdf:type predicate.

Shortcomings:
- the default list of common prefixes is relatively small, a lot more
  can be added since the unused ones won’t appear;
- the printer does not try to generate blank node property lists or
  collections.