Permuted list from its n-th element : shift

### 2.37.9 Permuted list from its n-th element : `shift`

`shift` takes as argument a list `l` and an integer `n` (by default `n`=-1).
`shift` rotates the list to the left if `n>0` or to the right if `n<0`. Elements leaving the list from one side are replaced by `undef` on the other side.
Input :

`shift([0,1,2,3,4])`

Output :

`[undef,0,1,2,3]`

Input :

`shift([0,1,2,3,4],2)`

Output :

`[2,3,4,undef,undef]`

Input :

`shift([0,1,2,3,4],-2)`

Output :

`[undef,undef,0,1,2]`