chore: add back `LitSet` improvements removed by revert. #372

Job	Run time
Unit tests	1m 9s
Unified Planning API	57s
Rustfmt and Clippy	47s
Solving	1m 52s
UP Integration Tests	3m 10s
Build - ${{ matrix.os }} - ${{ matrix.target }}	0s
Python Package (up_aries)	0s
tests	1s
Pre Release	0s
Release	0s
	7m 56s

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