Improve high level bindings to allow easier specification of integrality or bounds

It's very rare for a single variable type to have integer and non-integer instances.

Define a class which a Variable must be an instance of.

``` Haskell
data Type = Continuous | Binary | Integer

data Bound = Fin Int | Infinite

newtype Bounded = Bounded (Bound, Bound) 

class isVar a where
  hasType :: a -> Type
  hasBound :: a -> Bounded
```
