Implement `$struct$unnest()`

[etiennebacher]

Part of #998

pl$Series(name = "foo", values = 1)$to_struct()$struct$unnest()
shape: (1, 1)
┌─────┐
│ foo │
│ --- │
│ f64 │
╞═════╡
│ 1.0 │
└─────┘

@eitsupi I did as in #1002 but I'm not sure it's what you had in mind

[eitsupi]

I think this is a good implementation, but the Rd file is not generated. I think we can merge them for the time being and think about the documentation later.

I was imagining that it would be better to define the SeriesStruct_unnest and make it something like unnest=SeriesStruct_unnest

[etiennebacher]

the Rd file is not generated

It is generated, I just had to specify the @name otherwise it's named Series_unnest

[etiennebacher]

Ah no my bad, the usage section is wrong

[etiennebacher]

I'd rather find a solution here than coming back later for this.

[eitsupi]

Can we merge this?

[etiennebacher]

No the Usage section in the docs is still wrong