`id` is not generated with atomic stacking

[vspinu]

unnest(list(a = 1:3, b = 10:14), s(stack = "ix", as = "point", s(stack = "id")))
  ix point
1  a     1
2  a     2
3  a     3
4  b    10
5  b    11
6  b    12
7  b    13
8  b    14
> unnest(list(a = as.list(1:3), b = as.list(10:14)), s(stack = "ix", as = "point", s(stack = "id")))
  ix point point.id
1  a     1        1
2  a     2        2
3  a     3        3
4  b    10        1
5  b    11        2
6  b    12        3
7  b    13        4
8  b    14        5