Add feature sets

• Feature: add support for features from Product Line Engineering.
  • Add new definition rule feature_set_def to rule definition.
  • Add and, or, xor bodies to feature_set_def.
  • Add basic documentation to book.
• Feature: updates to builtin simple types.
  • Add the new type unsigned.
  • Add the new type binary.
  • Rename type iri_reference to iri.

Full Changelog: v0.1.40...v0.1.41