Would it be too warty to have :cast-with be the process option instead of :cast-fns?

[metasoarous]

It's what I keep thinking; but I feel funny about having only that one argument map differently (all the others map directly to the callout function arguments).

[metasoarous]

I'm going to do my best to avoid breaking changes, so if we do this at all, we'll do it as a duplicate option for consistency with the other process/slurp argument patterns.