Reconsider flatten_dfc() and flatten_dfr()

[hadley]

It's concerning that they both return the same results in this case:

library(purrr)

dfs <- list(c(a = 1), c(b = 2))
flatten_dfc(dfs)
#> # A tibble: 1 x 2
#>       a     b
#>   <dbl> <dbl>
#> 1     1     2
flatten_dfr(dfs)
#> # A tibble: 1 x 2
#>       a     b
#>   <dbl> <dbl>
#> 1     1     2

^{Created on 2020-04-16 by the reprex package (v0.3.0)}

[hadley]

Should either implement with vctrs or simply deprecate.