fix: restrict abv positivity extension to variable functions

[fpvandoorn]

• Restrict the evalAbv positivity extension to only apply to variable functions. Note: this extension is called on all applications, and uses type-class inference to see whether it applies
• Fix a bug in evalAbs, which would return .none, even when it could return .nonneg.
• Originally hoped that this would improve performance, but that does not seem to be the case.

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[github-actions[bot]]

PR summary e11cecf457

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

No declarations were harmed in the making of this PR! 🐙

You can run this locally as follows

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

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

The doc-module for script/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/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).

[fpvandoorn]

!bench

[leanprover-radar]

Benchmark results for e90beccdbac9614420d87c305cd945030ff34c32 against e11cecf457b7114a4d013dfb872273c41da6ee2d are in! @fpvandoorn

No significant changes detected.

[JovanGerb]

It's not possible for f to be a bvar. Note that whnf will panic if it sees a bvar as the head expression.

[JovanGerb]

Do we also want it to work for a family of functions? I.e. if f.getAppFn.isFVar?

Otherwise, looks good to me.