* Change is_zero_size to short-circuit internal all function when
  first element which isn’t zero sized is found.

* Change is_zero_size to use vector of str slices as stack.  There’s
  no need to allocate strings.  This also makes it possible to share
  the same stack as max_serialized_size_impl.

* Introduce Recursive error type to better encode error condition of
  is_zero_size.

* Change max_serialized_size’s count argument to NonZeroUsize to
  encode in type system that the count is in fact never zero.  In
  places where it might end up zero, the function short-circuits.