`SmallAtom` optimization #1641

Job	Run time
Wheel on macos-latest py-3.7	2m 28s
Checks	53s
Wheel on ubuntu-latest py-3.7	2m 33s
Wheel on windows-latest py-3.7	3m 47s
Run fuzz targets (macos-latest, 3.7)	38m 45s
Run fuzz targets (ubuntu-latest, 3.7)	24m 40s
coverage	7m 40s
Unit tests (macos-latest, 3.7)	3m 22s
Unit tests (ubuntu-latest, 3.7)	2m 11s
Unit tests (windows-latest, 3.7)	5m 0s
Upload to PyPI	25s
	1h 31m 44s

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