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ARROW-17366: [R] Support purrr-style lambda functions in .fns argument to across() (#14327)

Do not merge - putting this up as I made changes to the original after it had been approved and now the CI won't re-run so I'm testing it here before merging

Authored-by: Nic Crane <thisisnic@gmail.com>
Signed-off-by: Nic Crane <thisisnic@gmail.com>