Add runtime support for the State, Reader, and Fail effects that
were already defined in the type system. These effects can now be
used in effectful code blocks.
Changes:
- Add builtin_state and builtin_reader fields to Interpreter
- Implement State.get and State.put in handle_builtin_effect
- Implement Reader.ask in handle_builtin_effect
- Add Reader effect definition to types.rs
- Add Reader to built-in effects list in typechecker
- Add set_state/get_state/set_reader/get_reader methods
- Add 6 new tests for built-in effects
- Add examples/builtin_effects.lux demonstrating usage
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
