Improved unicode support in mutator, flattener, and more #2292

Job	Run time
tests (ubuntu-latest, unit, 3.8)	52s
tests (ubuntu-latest, unit, 3.12)	45s
tests (ubuntu-latest, integration, 3.8)	3m 5s
tests (ubuntu-latest, integration, 3.12)	3m 19s
tests (ubuntu-latest, tool, 3.8)	1m 5s
tests (ubuntu-latest, tool, 3.12)	54s
tests (windows-2022, unit, 3.8)	2m 8s
tests (windows-2022, unit, 3.12)	1m 51s
tests (windows-2022, integration, 3.8)	2m 46s
tests (windows-2022, integration, 3.12)	4m 6s
tests (windows-2022, tool, 3.8)	3m 22s
tests (windows-2022, tool, 3.12)	3m 31s
coverage	1s
	27m 45s

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