visitor context hooks

[ssube]

Add hooks to the visitor context so callers can be notified at various important times, including:

• item:
  • check (start)
  • skip (end - skip)
  • visited (start)
  • diff (mid - block)
  • error (end - fail)
  • pass (end - pass)
• rule:
  • visited (start)
  • error (end - fail)
  • pass (end - pass)

Along the way:

• push error formatting up to the hooks