Use multiple arguments instead of a tuple for pushforward and pullback function?

It seems annoying that the pushforward and pullback function accept tuples of co-tangents instead of multiple arguments. Is there a compelling reason for doing so or was this a design decision that could be changed? In my opinion the main annoyance is that one has to handle the case of tuples of length 1 in a special way (as e.g. in https://github.com/JuliaDiff/AbstractDifferentiation.jl/pull/51) (it also makes it impossible to work with actual single-argument functions that take a tuple as only argument but maybe this is not needed anyway). Arguably it is also cleaner to provide multiple arguments as, well, multiple arguments instead of a tuple.

Start by reviewing the existing pushforward and pullback interfaces and the tuple-handling example referenced in AbstractDifferentiation.jl pull request #51. Determine whether multiple arguments can replace tuple arguments while preserving support for single-argument tuple functions. Done means the API direction and implementation scope are agreed.