Update ERC-3009: fix dead links (#953)
* fix dead link in `erc-3009.md`
* fix broken link to safeERC20.sol in `erc-721.md`
* Duplicate SafeERC20.sol into erc-721
* Duplicate reference implementation into ERC-3009's assets dir
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Co-authored-by: Sam Wilson <sam@binarycake.ca>
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