[Merged by Bors] - feat(RefinedDiscrTree/Lookup): use parallellism in lookup

[JovanGerb]

This PR adds parallellism to the RefinedDiscrTree lookup. Previously, only constructing the root node of the discrimination tree used parallellism. But it turns out some of the child nodes can be pretty big too.

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[github-actions]

PR summary 3d8100bbc6

Import changes for modified files

No significant changes to the import graph

Import changes for all files

Files	Import difference

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Declarations diff

+ processPending

You can run this locally as follows

## summary with just the declaration names:
./scripts/pr_summary/declarations_diff.sh <optional_commit>

## more verbose report:
./scripts/pr_summary/declarations_diff.sh long <optional_commit>

The doc-module for scripts/pr_summary/declarations_diff.sh contains some details about this script.

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No changes to technical debt.

You can run this locally as

./scripts/reporting/technical-debt-metrics.sh pr_summary

• The relative value is the weighted sum of the differences with weight given by the inverse of the current value of the statistic.
• The absolute value is the relative value divided by the total sum of the inverses of the current values (i.e. the weighted average of the differences).

[adamtopaz]

Sorry for the delay...

[adamtopaz]

bors r+

[mathlib-bors]

Pull request successfully merged into master.

Build succeeded:

• ci (staging) / Build
• ci (staging) / Post-Build Step
• ci (staging) / Post-CI job
• ci (staging) / Lint style