Add CrossVMBridgeFulfillment base contracts

[sisyphusSmiling]

Closes: #166

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[sisyphusSmiling]

Update: Scoping the extension of ERC721 as a potential for linearization problems in ERC721 extensions - ref

[sisyphusSmiling]

Cherry picked 8eceb59 from #163 to fix CI

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.35%. Comparing base (02aab3f) to head (0ffd27c).
Report is 219 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #168      +/-   ##
==========================================
+ Coverage   86.19%   87.35%   +1.15%     
==========================================
  Files          18       18              
  Lines         884     1020     +136     
==========================================
+ Hits          762      891     +129     
- Misses        122      129       +7     

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[joshuahannan]

I'm certainly not an authority on Solidity, but this seems fine to me!

[sisyphusSmiling]

FYI @loic1 I've added exists(uint256) (bool) to the fulfillment base contracts. It will be necessary information in conjunction with isEscrowed

[loic1]

FYI @loic1 I've added exists(uint256) (bool) to the fulfillment base contracts. It will be necessary information in conjunction with isEscrowed

Gotcha, thanks! Added the change in TopShot contract