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This is another attempt on #11288 This allows for a `Stack` to have a parent stack (behind an `Arc`). This is being added to avoid constant stack copying in REPL code. Concretely the following changes are included here: - `Stack` can now have a `parent_stack`, pointing to another stack - variable lookups can fallback to this parent stack (env vars and everything else is still copied) - REPL code has been reworked so that we use parenting rather than cloning. A REPL-code-specific trait helps to ensure that we do not accidentally trigger a full clone of the main stack - A property test has been added to make sure that parenting "looks the same" as cloning for consumers of `Stack` objects --------- Co-authored-by: Raphael Gaschignard <rtpg@rokkenjima.local> Co-authored-by: Ian Manske <ian.manske@pm.me>