Protocol for guarded transitions, actions, etc.

Default implementations are provided for:

• BitString - Call callback module with string, context, and event
• Atom - Call callback module with atom, context, and event
• Function - Call function with context and event
• Tuple - Higher-order guard utilities:
  • {:and, [guard1, ...]}
  • {:or, [guard1, ...]}
  • {:not, guard}
  • {:in, query} - Delegates to Protean.Context.matches?/2