Fix syntax error for 0.13.x version of purs

In 0.13.0, the parser was changed so it wouldn't allow strings with
invalid Unicode. `\ffff` is now a syntax error. To get around this
issue, we move it to the FFI.

This isn't the only way to fix this problem, but it should still allow
the same behavior of the test without changing the spirit of the test.
If there's a different way to solve this problem that we'd prefer, we
should do that instead.

Author: joneshf

test/Test/Main.js

@@ -6,3 +6,4 @@ exports.thisIsNumber = 12;
 exports.thisIsString = "foobar";
 exports.thisIsArray = ["foo", "bar"];
 exports.thisIsObject = { foo: "bar" };
+exports.thisIsInvalidString = "\\\ffff";

test/Test/Main.purs

@@ -23,6 +23,7 @@ foreign import thisIsNumber :: Json
 foreign import thisIsString :: Json
 foreign import thisIsArray :: Json
 foreign import thisIsObject :: Json
+foreign import thisIsInvalidString :: String
 
 isTest :: Effect Unit
 isTest = do
@@ -120,7 +121,7 @@ toTest = do
 
 parserTest :: Effect Unit
 parserTest = do
-  assert ((isLeft (jsonParser "\\\ffff")) <?> "Error in jsonParser")
+  assert ((isLeft (jsonParser thisIsInvalidString)) <?> "Error in jsonParser")
   quickCheck' 10 roundtripTest
   where
   roundtripTest :: Gen Result