Fix normalization of partially-saturated folds

[travisbrown]

Noticed a couple of minor bugs while putting together a PR to remove this code, which was a temporary fix to emulate a bug in the Haskell implementation.

I don't want to actually remove this code until there's a new dhall-haskell release with the fix there, so I'm fixing the bug here for now.

The issue is pretty minor, and I think should only affect cases where you try to type-check a normalized expression with a partially-applied fold:

scala> import org.dhallj.syntax._
import org.dhallj.syntax._

scala> "List/fold Natural [1] Natural".parseExpr.flatMap(_.normalize.typeCheck)
res0: scala.util.Either[org.dhallj.core.DhallException,org.dhallj.core.Expr] = Left(org.dhallj.core.typechecking.TypeCheckFailure: Can't apply Natural)

After this change:

scala> import org.dhallj.syntax._
import org.dhallj.syntax._

scala> "List/fold Natural [1] Natural".parseExpr.flatMap(_.normalize.typeCheck)
res0: scala.util.Either[org.dhallj.core.DhallException,org.dhallj.core.Expr] = Right(∀(cons : (Natural → Natural → Natural)) → ∀(nil : Natural) → Natural)

Thankfully this whole terrible hack will be gone soon!

[travisbrown]

Not sure what's going on with Travis CI, which seems even more broken than usual today, but this build passed.