Artifact changes crafted manually: re-generated artifacts from within
@0x/contracts-erc20, and then copied them into @0x/contract-artifacts,
but only committed the changes that added devdoc.
Modified @0x/contracts-erc20/compiler.json to include devdoc in the
hopes that next time contract-artifacts are re-generated en masse, the
devdoc output will just come along for the ride.
Compiling ERC20 TypeScript wrappers after adding devdoc to the artifacts
revealed some inconsistencies in the types. ethereum-types'
DevdocOutput REQUIRED many fields which are not always present,
depending the devdoc contents itself. Relaxing the requirement had some
ramifications.