unnest_longer(), unnest_wider(), and hoist() should check that first argument is a data frame

[mgirlich]

This would give much better error messages when the user assumes that these functions work for lazy tables. Currently, the error messages do not help much:

library(tidyr)
library(dtplyr)

df <- tibble(
  x = 1:3,
  y = list(NULL, 1:3, 4:5)
) %>% 
  lazy_dt()

df %>% 
  hoist(y)
#> Error: object 'y' not found

df %>% 
  unnest_longer(y)
#> Error: `x` must be a vector, not a <dtplyr_step_first/dtplyr_step> object.

^{Created on 2021-11-16 by the reprex package (v2.0.1)}

Alternatively, these functions could be made generic (see #1071).