Add "SimpleT" t output form

This new output form allows easy piping to the decode function of the
tricu executable. Includes a new test for roundtrip evaluation of map,
compilation to tree calculus terms, and decoding back to a human
readable string.

test/Spec.hs

@@ -388,6 +388,9 @@ compilerTests = testGroup "Compiler tests"
   , testCase "Mapping and Equality" $ do
       res <- liftIO $ evaluateFile "./test/map.tri"
       res @?= Stem Leaf
+  , testCase "Map evaluation to String -> compilation -> string decoding" $ do
+      res <- liftIO $ evaluateFile "./test/string.tri"
+      decodeResult res @?= "String test!"
   ]
 
 propertyTests :: TestTree