Check for duplicate record type field names

[travisbrown]

Fixes #6:

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

scala> val Left(failure) = "{x: Natural, x: Natural}".parseExpr.flatMap(_.typeCheck)
failure: org.dhallj.core.DhallException = org.dhallj.core.typechecking.TypeCheckFailure: duplicate field: x

We follow the Haskell implementation in failing fast on duplicate field names even if there are invalid type errors:

scala> val Left(failure) = "{x: Natural, x: 0}".parseExpr.flatMap(_.typeCheck)
failure: org.dhallj.core.DhallException = org.dhallj.core.typechecking.TypeCheckFailure: duplicate field: x

Compare:

$ dhall --explain <<< "{x: Natural, x: 0}"
dhall: 
Error: Invalid input

(stdin):1:18:
  |
1 | {x: Natural, x: 0}
  |                  ^
duplicate field: x