Certora: Mutation testing and rule enhancements

[mmv08]

This pull request:

• Adds a setup for mutation testing by Alex from Certora. It includes the documentation on how to run it
• Based on the previous mutation testing iteration, I added three new rules:
  1. Verify that setFallbackHandler sets the new fallback handler correctly
  2. setup function correctly sets up a safe
  3. native token refund is always paid in execTransaction

[github-actions[bot]]

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I have read the CLA Document and I hereby sign the CLA

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1 out of 2 committers have signed the CLA.
:white_check_mark: @mmv08
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[coveralls]

Pull Request Test Coverage Report for Build 6272559599

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+1.2%) to 92.771%

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Totals
Change from base Build 6233603447:	1.2%
Covered Lines:	337
Relevant Lines:	357

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💛 - Coveralls