fix: Accurate tracking of slice capacities across blocks

[vezenovm]

Description

Problem*

Resolves #4202 and #2446 (comment)

Summary*

This PR brings back the capacity_tracker that was used by the fill internal slices pass and for merging nested slices (#3979). We now track the capacity of slices as we insert instructions. The capacity tracker has also been simplified as it no longer needs to handle nested slice.

The current strategy checks for previously saved store instructions and backtracks the instructions to see what is the capacity of a slice when it comes time to merge two slice values. This required an additional outer_block_stores map as we store_values only track stores within the current branch that is being inlined, and a get_slice_length method that ultimately did the recursive backtracking to determine the slice length. With the capacity tracker we already have the slice length once it comes time to perform a slice merger allowing us to remove outer_block_stores and get_slice_length.

Overall, the capacity tracker enables us to have more accurate tracking of slice capacities and gives us a more general strategy for tracking a slice capacity per instruction.

Additional Context

Documentation*

Check one:

• No documentation needed.
• Documentation included in this PR.
• [Exceptional Case] Documentation to be submitted in a separate PR.

PR Checklist*

• I have tested the changes locally.
• I have formatted the changes with Prettier and/or cargo fmt on default settings.

[github-actions]

Changes to circuit sizes

Generated at commit: 07e74d0c2d62778df1890f4d73074429b2f52ed8, compared to commit: 682b159504a00813f562fe08bd4bdaf8d2898b55

🧾 Summary (10% most significant diffs)

Program	ACIR opcodes (+/-)	%	Circuit size (+/-)	%
nested_array_in_slice	-639 ✅	-37.59%	-5,373 ✅	-42.82%

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Full diff report 👇

Program	ACIR opcodes (+/-)	%	Circuit size (+/-)	%
nested_array_in_slice	1,061 (-639)	-37.59%	7,175 (-5,373)	-42.82%

[vezenovm]

This is ready for review again @jfecher @TomAFrench @guipublic. Overall it simplifies how we track slice capacities and will let us generalize it to other passes if needed.

nested_array_in_slice -639 ✅ -37.59% -5,373 ✅ -42.82%

This reduction in constraint size provides a good example of the inefficiency that exists right now.

[jfecher]

👍