chore(Order/Iterate): avoid `to_dual existing`

[JovanGerb]

This PR adds a few uses of to_dual. Unfortunately many declarations cannot yet really be handled by to_dual because Monotone isn't supported yet.

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[github-actions[bot]]

PR summary 1825e179b3

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

- iterate_comp_le_of_le - le_iterate_of_le

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).

[bryangingechen]

Thanks! bors r+

[mathlib-bors[bot]]

Build failed:

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[bryangingechen]

bors r+

[mathlib-bors[bot]]

Pull request successfully merged into master.

Build succeeded:

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