Small flattener features #239

Job	Run time
fmt	11s
feature checks	1m 57s
clippy	51s
docs	44s
test stable (ubuntu-latest)	1m 19s
test stable --all-features (ubuntu-latest)	1m 52s
test 1.65 (ubuntu-latest)	39s
test stable (macos-latest)	3m 44s
test stable --all-features (macos-latest)	3m 55s
test 1.65 (macos-latest)	1m 57s
test stable (windows-latest)	3m 14s
test stable --all-features (windows-latest)	4m 45s
test 1.65 (windows-latest)	3m 5s
	28m 13s

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