a hack to get numeric-indexed deepprops working

- don't understand what the original insert_into
  idea for integer indices was
- we don't convert anything to integer and expect
  all the keys to be strings (even if the value itself
  is integer)
- to fix that quickly, we included a deepprops library
  to make nested proplists for us

WARNING: This is not intended to be a definitive solution,
it's just a hack to bring up the expected functionality
to project that relies on it.