API: Implement vim_eval

The vimscript object conversion function was adapted from the version found in
'if_py_both.h' in the upstream source. It was also required to re-add the
`dict_lookup` function that was lost during the initial import.

src/eval.c

@@ -5261,6 +5261,11 @@ dictitem_T *dict_lookup(hashitem_T *hi)
 }
 #endif
 
+dictitem_T * dict_lookup(hashitem_T *hi)
+{
+    return HI2DI(hi);
+}
+
 /*
  * Return TRUE when two dictionaries have exactly the same key/values.
  */