Complete porting for MACI gatekeepers (#31)

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## Description
This PR adds the `GitcoinPassportExcubia`,
`ZKEdDSAEventTicketPCDExcubia` (prev Zupass) and, `HatsExcubia`. During
the porting process, certain interfaces were extended and controls and
methods generalised. The code coverage is 100%.

Also, this PR introduces the concept of `trait` aka the specific type of
an Excubia contract. For example, `SemaphoreExcubia` has trait
`Semaphore` and so on. This will make easy to discriminate and query
multiple Excubiae sharing the same characteristics.

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## Related Issue(s)
closes #18
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## Checklist

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-   [x] My code follows the style guidelines of this project
-   [x] I have performed a self-review of my code
- [x] I have commented my code, particularly in hard-to-understand areas
-   [x] My changes generate no new warnings
- [x] I have run `yarn format` and `yarn compile` without getting any
errors
- [x] I have added tests that prove my fix is effective or that my
feature works
-   [x] New and existing unit tests pass locally with my changes