Group Hash and DiversifyHash refactoring. Also fix an error in the definition of set difference.
Group Hash and DiversifyHash refactoring. Also fix an error in the definition of set difference.

Signed-off-by: Daira Hopwood <cd7d43d5556a38ce626d53826fddc46adc597539@jacaranda.org>

zcash/zips,zcash/zips,zcash/zips,zcash/zips