Builder: profile NFT

[a17]

Initial src/core/Builder.sol contract implementation.

Minimal logic for investing funds and paying bounty.

Requirements

• [x] ERC721
• [ ] enumerable
• [ ] off-chain metadata baseUri
• [ ] non-transferable, single whitelisted transfers
• [ ] token data
  • invested
  • bountyPending
  • bountyGot
• [ ] counter
  • invested
  • bountyPending
  • bountyPaid
• [ ] invest(address asset, uint amount)
• [ ] addPendingBounty(uint tokenId, uint amount)
• [ ] payBounty(uint tokenId, uint amount)