Add first ERC721 version + Solidity test suite

[sisyphusSmiling]

Stacked on: #9

Closes: #2 #3
Related: #6

Description

• Introduces the base ERC721 (minus random minting as TODO), accepting any IERC20 implementation as payment for minting
• Adds the canonical WFLOW contract as a local dependency
• Adds an example ERC20 for testing purposes

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For contributor use:

• Targeted PR against master branch
• Linked to Github issue with discussion and accepted design OR link to spec that describes this work.
• Code follows the standards mentioned here.
• Updated relevant documentation
• Re-reviewed Files changed in the Github PR explorer
• Added appropriate labels

[joshuahannan]

Seems fine to me, but might be good to get someone who is more of a solidity expert to look at this