Derive instances for Existentials

[jakzale]

See #79.

Ord and Eq instances for Some... could be automatically derived.

And we could use the some package for Existential.hs.

instance Eq (SomeUni f) where
    Some u1 == Some u2 = withGeqUni u1 u2 False True

instance Ord (SomeUni f) where

 Some (UniConst Vector v1) == Some (UniConst Vector v2) = v1 == v2
 Some (UniConst Bool _)    == _                         = False
 Some (UniConst Field _)   == _                         = False
 Some (UniConst Vector _)  == _                         = False