Refactor and advance core framework capabilities

[0xjei]

This PR extends the core framework's functionality based on #5 conversations and more.

• There will be a Base and Advanced gatekeeper division with reusable Checker contracts. This is a huge breaking change compared to previous prototype implementation on zk-kit.
• The usage of a nested mapping mapping(address => mapping(address => bool)) can make us able to simulate different deployed instances of Excubiae Gatekeeper and maintain a reference like 1 contract <> N independent states (who already passed the gatekeeper). This will allow us to get rid of the non-determinism in deployments and maintain a structure where navigating and managing the different gatekeepers smoothly.
• Making the checks to return a boolean.

The change will allow for other changes, as:

• [x] Checker independent contract instead of check() method
• [x] Base and Advanced separation of concerns for Excubia and Checker contracts
• [x] Common mappings to handle AlreadyPassed checks & logic
• [x] Use mapping of mapping to handle 1 contract <> N independent gatekeeper states
• [x] Checks must return a bool
• [x] Separation of concerns for Excubia extensions (FreeForAllExcubia)
• [x] Apply new naming and conventions
• [x] Pattern: balance _internal vs external methods
• [x] Full code coverage (create a fixture for base & advanced for testing purposes)
  • [x] Configs
  • [x] Base
    • [x] BaseChecker
    • [x] BasePolicy
    • [x] BaseVoting (integration)
  • [x] Advanced
    • [x] AdvancedChecker
    • [x] AdvancedPolicy
    • [x] AdvancedVoting (integration)
• [x] Refactoring
• [x] Review code comments and other in-code documentation