Fix toString's handling of text literals, etc.

[travisbrown]

The implementation of toString (taking an expression and writing it back to Dhall code as a string) is currently pretty rough and needs some attention in general, but in particular it doesn't serialize text literals containing newlines correctly. toString hasn't been a high priority for me so far, but the newline issue at least needs to be fixed before the 0.1.0 release.

[travisbrown]

This particular issue and some related toString problems are fixed in 02bd091. It's now possible to load the unnormalised Prelude, print it with toString, parse it again, and get the same result:

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

scala> val Right(prelude) = "./dhall-lang/Prelude/package.dhall".parseExpr.flatMap(_.resolve)
prelude: org.dhallj.core.Expr = {`Bool` = ...

scala> val Right(fromToString) = prelude.toString.parseExpr
fromToString: org.dhallj.core.Expr = {`Bool` = ...

scala> prelude.hash
res0: String = ce1b587ca440b150fdc5d892f6a0a54873d0f6ec89a4fcff5b4d698a9ea45189

scala> fromToString.hash
res1: String = ce1b587ca440b150fdc5d892f6a0a54873d0f6ec89a4fcff5b4d698a9ea45189

scala> prelude.diff(fromToString)
res2: Option[(Option[org.dhallj.core.Expr], Option[org.dhallj.core.Expr])] = None

The toString implementation is still a terrible mess, but I think this round of fixes unblocks 0.1.0.