sugar for data and other improvements

- parse types in let (everything but parser was there)
- add sugar for `data`
- move `joinBy` to prelude
- fix highlighting for char in vscode
- better errors for missing imports

newt-vscode/syntaxes/newt.tmLanguage.json

@@ -26,11 +26,14 @@
         { "include": "source.js" }
       ]
     },
+    {
+      "name": "string.newt",
+      "match": "'(.|\\\\.)'"
+    },
     {
       "name": "string.newt",
       "begin": "\"",
       "end": "\""
     }
-
   ]
 }