[RPKG] Remove duplicated compute IOI hash implementations, more cleanup

[RDIL]

• 200% reduction in number of duplicate implementations of IOI hash calculation!
• Use modern for loop syntax in some places.
• Mark some things as constants for additional optimization.
• Move dev functions into an experimental folder.
• More c-style casts to c++ style casts.

[AnthonyFuller]

Before I review in about 20 minutes. Could you force push to rename the commit to follow the (current) style?

[github-actions[bot]]

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• rpkg.zip
• rpkg-src.zip