Disallow with changing Optional record type (#2501)

See: dhall-lang/dhall-lang#1332

The standard already required this, but the Haskell implementation was
not correctly conforming to the standard.  Fixing this not only
simplifies the Haskell code but also makes the code more greatly
resemble the corresponding standard judgement.

Author: Gabriella439

dhall/src/Dhall/TypeCheck.hs

@@ -1315,16 +1315,12 @@ infer typer = loop
 
                   VOptional _O' -> do
                     case ks of
-                      WithQuestion  :| [] -> do
+                      WithQuestion  :| _ -> do
                         tV' <- loop ctx v
                         if Eval.conv values _O' tV'
                           then return (VOptional _O')
                           else die OptionalWithTypeMismatch
 
-                      WithQuestion :| k₁ : ks' -> do
-                        tV' <- with _O' (k₁ :| ks') v
-                        return (VOptional tV')
-
                       WithLabel k :| _ -> die (NotAQuestionPath k)
 
                   _ -> die (NotWithARecord e₀ (quote names tE')) -- TODO: NotWithARecordOrOptional