refactor(contracts): inline witnesses into the simulators

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Types of changes

What types of changes does your code introduce to OpenZeppelin Midnight Contracts?
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• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation Update (if none of the other choices apply)

This PR proposes changes on the structure of the witnessess through the repo, inspired by the midnight-apps repo structure.

PR Checklist

• I have read the Contributing Guide
• I have added tests that prove my fix is effective or that my feature works. See GUIDELINES.md#testing for more information.
• I have added documentation for new methods or changes to existing method behavior. See GUIDELINES.md#documentation for more information.
• CI Workflows Are Passing

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