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
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Co-authored-by: Raphael Gaschignard <rtpg@rokkenjima.local>
Co-authored-by: Ian Manske <ian.manske@pm.me>
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