remove NodePtr hack and fix-up benchmark-clvm-cost #1653

Job	Run time
Wheel on macos-latest py-3.7	4m 32s
Wheel on ubuntu-latest py-3.7	3m 1s
Checks	48s
coverage	7m 40s
Wheel on windows-latest py-3.7	3m 26s
Run fuzz targets (macos-latest, 3.7)	47m 24s
Unit tests (macos-latest, 3.7)	8m 30s
Run fuzz targets (ubuntu-latest, 3.7)	24m 8s
Unit tests (ubuntu-latest, 3.7)	3m 9s
Unit tests (windows-latest, 3.7)	5m 45s
Upload to PyPI	24s
	1h 48m 47s

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