Mostly-fix Trac #2595: updates for existentials
Ganesh wanted to update records that involve existentials. That seems reasonable to me, and this patch covers existentials, GADTs, and data type families. The restriction is that The types of the updated fields may mention only the universally-quantified type variables of the data constructor This doesn't allow everything in #2595 (it allows 'g' but not 'f' in the ticket), but it gets a lot closer. Lots of the new lines are comments!