Re-export `SNat` from `base`

[martijnbastiaan]

SNat is part of base-4.18.0.0 and later. Up to this point, we've defined our own version:

data SNat (n :: Nat) where
  SNat :: KnownNat n => SNat n

base's version is a bit more complex:

newtype SNat (n :: Nat) = UnsafeSNat Natural
type role SNat nominal

pattern SNat :: forall n. () => KnownNat n => SNat n
pattern SNat <- (knownNatInstance -> KnownNatInstance)
  where SNat = natSing
{-# COMPLETE SNat #-}

data KnownNatInstance (n :: Nat) where
  KnownNatInstance :: KnownNat n => KnownNatInstance n

knownNatInstance :: SNat n -> KnownNatInstance n
knownNatInstance sn = withKnownNat sn KnownNatInstance

Could we re-export for newer bases and expect everything to JustWork(tm)?