Type for rtl values

[jkopanski]

RTL values can be Valid or Invalid. Any operation involving Invalid value yields another Invalid.

Valid values can are characterized by its type (Ty or RTy). It would be nice to have basic set of typeclasses instances for those like Num or Bounded.

[jkopanski]

While doing complex type for rtl expression like:

data Value :: Ty -> * where
  Value :: Sing t -> Int -> Value t

and later using it for expression evaluation, it occurred to me that this is much overkill.
Expressions carry type information that can be used to alter the result itself and typechecking is making sure that we won't actually need to clip and roll the resulting values.

I've decided to go with simple Int as expression value and use Maybe for its possible invalidity.