* Move Ancetre() from path-boolop to repr-sorting.
* Rename and document functions in repr-sorting.
* Add flattened(), a non-destructive analog of sp_flatten().
* Move improved_cut() from boolean-subitems to path-boolop.
* Cleanup sp_pathvector_boolop().

Furthermore, add an additional flatten in the shape builder to remove
spurious filaments in livarot output.

LPE reference files are updated to reflect the fact that flattening no
longer uses an arbitrary threshold of 1.0, but is now scale-aware.