Allow unsorted (but still "continuous") index for packing

[hombit]

In src/nested_pandas/series/packer.py we frequently assume index to be sorted, for example here:

nested-pandas/src/nested_pandas/series/packer.py

Line 131 in a96a508

def pack_sorted_df_into_struct(df: pd.DataFrame, name: str | None = None) -> pd.Series:

nested-pandas/src/nested_pandas/series/accessor.py

Line 231 in a96a508

return pack_sorted_df_into_struct(flat)

We should make it more flexible and allow "continuous" index.

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[hombit]

It is partially replaced with #161, so closing for now