[WIP] Pollard functionality

[Davidson-Souza]

This [WIP] PR implements the Pollard data structure and associated logic.

TODO list:

• [x] Implement basic Pollard
• [x] Implement Modify as in the Go lib
• [x] Make it pass our tests from Stump modify
• [x] Implement proving logic
• [x] Test multiple addition and deletions
• [ ] Implement caching logic (non-full Pollard)
• [ ] Maybe fuzz it to find edge cases and unhandled exceptions
• [ ] Implement undo