- Rename pkg/interpreter to pkg/runtime
- Move ReduceOnce to new pkg/normalorder package
- Convert standalone functions (Substitute, Rename, GetFree, IsFree)
to receiver methods on concrete expression types
- Change Set from pointer receivers to value receivers
- Update all references from interpreter to runtime terminology
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
