feat: Implement sorted flags for struct series

[JakubValtar]

This PR implements sorted flags for struct series.

import polars as pl
s = pl.Series("a", [{"x": 3 }, {"x": 1 }, {"x": 2 }]).sort()
assert s.flags["SORTED_ASC"]

It removes a duplicate StructChunked::arg_sort function, it exists in ChunkSort<StructType> for StructChunked with the same name and implementation.

Closes #13484

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.89%. Comparing base (5e51218) to head (0cee9a7).
Report is 17 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #21290      +/-   ##
==========================================
+ Coverage   79.82%   79.89%   +0.06%     
==========================================
  Files        1596     1596              
  Lines      228565   228562       -3     
  Branches     2608     2608              
==========================================
+ Hits       182462   182618     +156     
+ Misses      45507    45348     -159     
  Partials      596      596              

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[ritchie46]

Why was this removed? Was it unused?

[JakubValtar]

It seems like this method was left over when ChunkSort was implemented for structs in #17522. A method with the same name, signature and implementation was added in ChunkSort impl and now some calls resolve to this one and some to the ChunkSort one. I removed it, because it is duplicate code, confusing to read, and a potential footgun if the implementations are not kept in sync.

This is the situation:

impl StructChunked {
    // <<< A >>> only used in `sort_with` below and one other place
    pub(crate) fn arg_sort(&self, options: SortOptions) -> IdxCa {
        let bin = _get_rows_encoded_ca(...).unwrap(); // same impl as B, just `get_row_encoded` inlined
        bin.arg_sort(Default::default())
    }
}

impl ChunkSort<StructType> for StructChunked {
    fn sort_with(&self, mut options: SortOptions) -> ChunkedArray<StructType> {
        ...
        let idx = self.arg_sort(options); // Looks like it calls B, but actually calls A
        ...
    }

    fn sort(&self, descending: bool) -> ChunkedArray<StructType> {
        self.sort_with(...) // Also calls A through `sort_with`
    }

    // <<< B >>> used in `T: ChunkSort` generic contexts
    fn arg_sort(&self, options: SortOptions) -> IdxCa {
        let bin = self.get_row_encoded(options).unwrap();
        bin.arg_sort(Default::default())
    }
}

[ritchie46]

Right, now I see.