Add frontend for rules

Rules can currently only be added reliably through the (low-level) generated API. The user should be able to specify them as well

Tasks include:

• Create concept, how rules can be represented as text
  • This may lead to types being either deprecated or deleted
• Extend parser to parse this textual representation
• Extend pretty-printer to print this textual representation
• Extend marshalling tests to include rules
• Extend documentation