Make VEC_GET take an index along with the type

Since most use cases require indexing the data directly, and the macro
implies you're retrieving data from it, may as well take the index.

If you wanted a pointer to that data, &VEC_GET(vec, index, type) works
just fine.

runtime/sys.c

@@ -40,7 +40,7 @@ void sys_cleanup(sys_t *sys)
   // Iterate through each cons currently allocated and free them
   for (size_t i = 0; i < VEC_SIZE(&sys->conses, lisp_t **); ++i)
   {
-    lisp_t *allocated = VEC_GET(&sys->conses, lisp_t *)[i];
+    lisp_t *allocated = VEC_GET(&sys->conses, i, lisp_t *);
     switch (get_tag(allocated))
     {
     case TAG_CONS: