Change prefix stack to List[String]

[jackkoenig]

This allows for sharing of the prefix stacks between Data, and removes a
boxing per prefix stack element that is no longer necessary.

My intuition is that this should reduce memory by quite a bit and thus improve performance, but I'd like to try it out on complex designs before we merge.

This should improve memory and performance for a few reasons:

1. construction_prefix for each Data is now free (it formerly copied from ArrayBuffer to List)
2. Structural sharing of common prefix stacks and intermediate prefix stacks
3. No more Either in prefix stack, removes an object allocation per stack element

While we do have to copy the prefix stack when we reverse for naming, this is a copy on naming of Data rather than on construction. In theory, most Data should not be named, but due to the fact that every HasId adds itself to the ids of its parent module, every Data does currently get named. Once that latter issue is resolved, this will result in far fewer copies of the prefix stack. It is still better with this change because the copies can be GCed right away rather than sticking around being referred to by each Data. We can also consider memoizing the prefix stack reversals.

Contributor Checklist

• [NA] Did you add Scaladoc to every public function/method?
• Did you add at least one test demonstrating the PR?
• Did you delete any extraneous printlns/debugging code?
• Did you specify the type of improvement?
• Did you add appropriate documentation in docs/src?
• Did you state the API impact?
• Did you specify the code generation impact?
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Type of Improvement

• performance improvement

API Impact

No impact

Backend Code Generation Impact

No impact

Desired Merge Strategy

• Squash

Release Notes

Reduce memory overhead from better naming improvements in v3.4.0

Reviewer Checklist (only modified by reviewer)

• Did you add the appropriate labels?
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[azidar]

As is LGTM. I don't think you need a test because you aren't changing any behavior.