make robust to changes in {dplyr}

[brshallo]

Initial code for R/pairwise.R is copied from @carlomedina https://github.com/carlomedina/dplyr/blob/carlomedina/pairwise/R/across.R . Currently the approach modifies or copies in a lot of the backend / unexported functions from {dplyr}. If these change pairwiser() may break.

It may make sense to write such that only depends on exported functions from {dplyr} or in such a way that can feel good is not too fragile.

Also, with the current set-up, if we wanted to put the package on CRAN, we would likely need to copy the functions in from dplyr directly (rather than using dplyr:::fun().