Fresh variable generation API #3

Currently, the API allows one to only quantify over inputs via its type signature. For example, we quantify over all platform sized integers Int in the following snipped:

{-# ANN example0 (Theory embeddings) #-}
example0 :: Int -> SMT.Bool

A more user-friendly and robust would be to expose a way to create symbolic variables directly to the user via a simple API. I.e. we would expose the following base function:

symbolic :: Primitive a => Identifier -> a

The Identifier is likely a type alias for SMT.Integer and will evaluation should halt if it is ever left symbolic. With this, we can just implement a typeclass similar to Arbitrary from QuickCheck.