Expose hedgehog function for testing

[micahhahn]

This PR exposes a new function from the Test module called hedgehog. This function is almost identical to the fuzz function except that we accept the Gen type that underlies the Fuzz opaque type:

hedgehog :: (Stack.HasCallStack, Show a) => Hedgehog.Gen a -> Text -> (a -> Expectation) -> Test

This allows consumers of the library to use the raw powerful Hedgehog combinators if they need to instead of our Elm-like wrappers.

Why do we need this?

I'm trying to write a Fuzzer for a recursive type that is an extension of a JSON object (for json-render):

newtype StatePath = StatePath Text

data Exp
  = Object (Map Text Exp)
  | Array [Exp]
  | String Text
  | Number Scientific
  | Bool Bool
  | BindState StatePath

A first attempt at a fuzzer might look like:

fuzzExp :: Fuzzer Exp
fuzzExp = 
  Fuzz.oneOf
    [ map (Map.fromList >> Object) (Fuzz.list (Fuzz.tuple (Fuzz.text, fuzzExp))),
      map Array (Fuzz.list fuzzExp)
      map String Fuzz.text,
      map (fromFloatDigits >> Number) Fuzz.float,
      map Bool Fuzz.bool, 
      map (StatePath >> BindState) Fuzz.Text
    ]

This of course will cause an infinite recursion.