feat(Logic/Function): add Lawvere fixed-point theorem

[JohnnyTeutonic]

Adds Lawvere's fixed-point theorem.

This is the classical diagonal argument that generalizes \cantor_surjective\ and \cantor_injective\ (both already in Mathlib). The proof is a two-line term-mode construction.

[github-actions]

PR summary 60d0c4b32a

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

+ exists_fixed_point_of_surjective

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 scripts/declarations_diff.sh contains some details about this script.

---

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).

[vihdzp]

I like this :)

[JohnnyTeutonic]

Ready for merge queue?