feat(rust, python): add .struct.field() expression expansion

[cmdlineluser]

Update: This was a much more complex problem than I realised - sorry for the noise.

[cmdlineluser]

Hmm - yes, so there is quite a bit more to this than it seemed at first look.

pl.col().struct.field("*") is only the simplest form, as schema.get(col) gives you the field names.

However, if we chain some calls, we can see it's rather more complex:

pl.col("foo").struct.field("*").struct.field("*").struct.field("C")

expr.into_iter() yields the following order:

pl.col("foo").struct.field("*").struct.field("*").struct.field("C")
pl.col("foo").struct.field("*").struct.field("*")
pl.col("foo").struct.field("*")
pl.col("foo")

As the parent may be another struct, all ancestors need to be expanded in order to access the field names.

Chains consisting of multiple "*" calls also brings up the topic of name clashing. It seems like a default behaviour of automatically prefixing the "current path" to the returned name would be needed in order to prevent this.