canonical type for type "function" isn't "bytes24"

[esaulpaugh]

I believe, based on the ethereum ABI spec and confirmed using Remix, that the function selector should be computed using the string "function" for function types and that "function" shall be used in the ABI json as well, and not "bytes24", despite function types having an identical encoding to bytes24 in the calldata. I believe 4byte is also incorrectly using "bytes24" to compute the selector.