With an existing column, I want to sequence a list of dates to each row e.g.

```
tibble(x = c("a", "b", "c"))

# A tibble: 3 x 1
  x    
  <chr>
1 a    
2 b    
3 c 

```

How do I tweak mutate to give me this output?

```
# A tibble: 9 x 2
  x     dates     
  <chr> <date>    
1 a     2021-01-01
2 a     2021-01-02
3 a     2021-01-03
4 b     2021-01-01
5 b     2021-01-02
6 b     2021-01-03
7 c     2021-01-01
8 c     2021-01-02
9 c     2021-01-03

```