Error on unnamed values if names_sep isn't provided

[DavisVaughan]

Closes #1367

So now we get:

library(tidyr)

df <- tribble(
  ~x, ~y,
  1, c(11, 12, 13),
  2, 21
)
df |> unnest_wider(y)
#> Error in `unnest_wider()`:
#> ! Can't unnest elements with missing names.
#> ℹ Supply `names_sep` to generate automatic names.

Also fixes an issue where partially unnamed elements weren't being patched with automatic names. Instead they were getting name-repaired by vctrs, which ended up generating a strange (and not useful) combination of names.

library(tidyr)

df <- tribble(
  ~x, ~y,
  1, c(11, b = 12, 13),
  2, 21
)

# before
df |> unnest_wider(y, names_sep = "_")
#> New names:
#> • `` -> `...1`
#> • `` -> `...3`
#> # A tibble: 2 × 5
#>       x y_...1   y_b y_...3   y_1
#>   <dbl>  <dbl> <dbl>  <dbl> <dbl>
#> 1     1     11    12     13    NA
#> 2     2     NA    NA     NA    21

# after
df |> unnest_wider(y, names_sep = "_")
#> # A tibble: 2 × 4
#>       x   y_1   y_b   y_3
#>   <dbl> <dbl> <dbl> <dbl>
#> 1     1    11    12    13
#> 2     2    21    NA    NA

^{Created on 2023-01-12 with reprex v2.0.2.9000}

[hadley]

This looks much better.

Will need to check for any revdep breakage.

[DavisVaughan]

"a324805f-c91c-43fc-9b50-405affcbfe24" - No failures from this!