Implement $meta$tree_format()

[etiennebacher]

library(polars)

e = (pl$col("foo") * pl$col("bar"))$sum()$over(pl$col("ham")) / 2
e$meta$tree_format()
#>     binary: //    
#> 
#>        |              |       
#> 
#>     lit(2.0)          window      
#> 
#>                       |               |        
#> 
#>                       col(ham)        sum          
#> 
#>                                       |        
#> 
#>                                       binary: *    
#> 
#>                                       |                |       
#> 
#>                                       col(bar)         col(foo)

---

@sorhawell I also did some minor refactoring for some DataFrame functions on Rust side, following your explanations in #399

[sorhawell]

Looks very good