Add support for `across()` in `arrange()`

[nathaneastwood]

At a minimum, the following tests should pass

df <- data.frame(x = c(1, 3, 2, 1), y = c(4, 3, 2, 1))
expect_identical(
  df %>% arrange(across()),
  df %>% arrange(x, y),
  info = "arrange() supports across() (#88): 1"
)
expect_identical(
  df %>% arrange(across(.fns = desc)),
  df %>% arrange(desc(x), desc(y)),
  info = "arrange() supports across() (#88): 2"
)
expect_identical(
  df %>% arrange(across(x)),
  df %>% arrange(x),
  info = "arrange() supports across() (#88): 3"
)
expect_identical(
  df %>% arrange(across(y)),
  df %>% arrange(y),
  info = "arrange() supports across() (#88): 4"
)