Implement keccak256 operator and softfork #1965

Job	Run time
Wheel on macos-13 py-3.8	2m 29s
Run fuzz targets (macos-13, 3.8)	19m 55s
Wheel on ubuntu-latest py-3.8	2m 53s
Checks	45s
Run fuzz targets (ubuntu-latest, 3.8)	14m 14s
Wheel on windows-latest py-3.8	1m 55s
Unit tests (macos-13, 3.8)	5m 8s
coverage	8m 22s
Unit tests (ubuntu-latest, 3.8)	3m 28s
Unit tests (windows-latest, 3.8)	6m 16s
Upload to PyPI	5s
	1h 5m 30s

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