Investigate type-checking performance

[travisbrown]

Right now the normalized Prelude type-checks ~instantaneously:

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

scala> val path = "./dhall-lang/Prelude/package.dhall"
path: String = ./dhall-lang/Prelude/package.dhall

scala> val Right(prelude) = path.parseExpr.flatMap(_.resolve)
prelude: org.dhallj.core.Expr = {`Bool` = {and = let ...

scala> val normalized = prelude.normalize
normalized: org.dhallj.core.Expr = {`Bool` = {and = ...

scala> normalized.typeCheck
res0: Either[org.dhallj.core.typechecking.TypeCheckFailure,org.dhallj.core.Expr] = Right(...

If you don't normalize it, though, it takes a little over a minute:

scala> prelude.typeCheck
res1: Either[org.dhallj.core.typechecking.TypeCheckFailure,org.dhallj.core.Expr] = Right(...

It does produce the correct result, but something is obviously wrong. I haven't really looked into this yet, but I'm assuming it's because it's type-checking the same imported trees over and over.