Deprecate gather.rset

[juliasilge]

Closes #140

This PR deprecates the gather.rset method and advises users to use pivot_longer() directly:

library(rsample)
library(tidyverse)
cv_obj <- vfold_cv(mtcars, v = 10)
cv_obj$lm_rmse <- rnorm(10, mean = 2)
cv_obj$nnet_rmse <- rnorm(10, mean = 1)

cv_obj %>%
  select(-splits) %>%
  pivot_longer(-id)
#> # A tibble: 20 x 3
#>    id     name       value
#>    <chr>  <chr>      <dbl>
#>  1 Fold01 lm_rmse   2.69  
#>  2 Fold01 nnet_rmse 1.99  
#>  3 Fold02 lm_rmse   2.60  
#>  4 Fold02 nnet_rmse 0.554 
#>  5 Fold03 lm_rmse   1.28  
#>  6 Fold03 nnet_rmse 0.856 
#>  7 Fold04 lm_rmse   2.20  
#>  8 Fold04 nnet_rmse 2.84  
#>  9 Fold05 lm_rmse   2.16  
#> 10 Fold05 nnet_rmse 2.07  
#> 11 Fold06 lm_rmse   1.90  
#> 12 Fold06 nnet_rmse 0.524 
#> 13 Fold07 lm_rmse   3.20  
#> 14 Fold07 nnet_rmse 0.0222
#> 15 Fold08 lm_rmse   0.954 
#> 16 Fold08 nnet_rmse 0.140 
#> 17 Fold09 lm_rmse   0.204 
#> 18 Fold09 nnet_rmse 0.318 
#> 19 Fold10 lm_rmse   2.33  
#> 20 Fold10 nnet_rmse 0.131

^{Created on 2021-04-15 by the reprex package (v2.0.0)}

For now, let's not create a new collect_ function to get these kinds of results and see if this is a requested feature from users.

[hfrick]

Looks good! I noticed that the lifecycle vignette also suggests updating the examples and the tests. That might be a nice addition so that people see the select(-split) in action?

[juliasilge]

Great idea @hfrick! I don't think I want to change the tests for this at this stage of only warning (and I don't want to test tidyr functionality) but adding the example is a good call.

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