[Rd] aggregate.formula and pipes

[Gabor Grothendieck]

Because aggregate.formula has a formula argument but the generic
has an x argument neither of these work:

  mtcars |> aggregate(x = mpg ~ cyl, FUN = mean)
  mtcars |> aggregate(formula = mpg ~ cyl, FUN = mean)

This does work:

  mtcars |> stats:::aggregate.formula(formula = mpg ~ cyl, FUN = mean)

Suggest that aggregate.formula be exported.

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