Fix Natural/fold for GHCJS bindings (#985)

Fixes dhall-lang/dhall-lang#579

`Natural/fold` was misbehaving on GHCJS due to the use of `Natural`
arithmetic (for some reason).  This is a problem I've seen and fixed
before, but needed to be fixed again after the migration to the new
`Dhall.Eval` normalization engine.

The fix is easy: use `Integer` instead of `Natural` for the accumulator
of the fold and the bug disappears.

Author: Gabriella439

dhall/src/Dhall/Eval.hs

@@ -368,7 +368,7 @@ eval !env t =
                             VHLam (Typed "zero" natural) $ \zero ->
                               let go !acc 0 = acc
                                   go  acc n = go (vApp succ acc) (n - 1)
-                              in go zero n
+                              in go zero (fromIntegral n :: Integer)
                           n ->
                             VHLam (NaturalFoldCl n) $ \natural -> VPrim $ \succ -> VPrim $ \zero ->
                               VNaturalFold n natural succ zero