Add new `Case` constraint which is more general than `Table Expr`

[shane-circuithub]

The Case constraint is used for functions like bool, case_, maybeTable, nullable, all of which ultimately compile down to a PostgreSQL CASE statement. Case has two instances: an overlapping Table Expr a => Case a instance, and a Case b => Case (a -> b) instance, that allows expressions like maybeTable id (+) which would not have been possible for.