Derive `Prim` instances for generated datatypes

[jorisdral]

See https://hackage-content.haskell.org/package/primitive-0.9.1.0/docs/Data-Primitive-Types.html#t:Prim

For examples, we can derive Prim instances for newtypes around primitive C types, like CInt:

typedef int MyInt

newtype MyInt { un_MyInt :: CInt }
  deriving newtype Prim -- this deriving statement would be the new feature

The primitive package is already in the dependency stack too, by the way