I have a `tibble` with one column being a `list` column, *always* having two numeric values named `a` and `b` (e.g. as a result of calling `purrr:map` to a function which returns a *list*), say:

```
df <- tibble(x = 1:3, y = list(list(a = 1, b = 2), list(a = 3, b = 4), list(a = 5, b = 6)))
df
# A tibble: 3 × 2
  x          y
 <int>     <list>
1     1 <list [2]>
2     2 <list [2]>
3     3 <list [2]>

```

How do I separate the list column `y` into two columns `a` and `b`, and get:

```
df_res <- tibble(x = 1:3, a = c(1,3,5), b = c(2,4,6))
df_res
# A tibble: 3 × 3
  x     a     b
 <int> <dbl> <dbl>
1     1     1     2
2     2     3     4
3     3     5     6

```

Looking for something like `tidyr::separate` to deal with a list instead of a string.