Split prepare(), perform(), tidy()

[jonthegeek]

See jonthegeek/nectar#33
Even if that doesn't work out, split up the pieces of both the overall call() function (if that survives) and the individual operation (path) functions. That way users can add things from httr2 as desired (throttling, parallel perform, etc) while still being able to take advantage of the recognizable process. Note that most likely the second and third steps for each function will be perform.thisOperation() and tidy.thisOperation() and then it's always prep-function-that-outputs-classes-req |> perform() |> tidy().