probably want `length(::GtkListStore, ::GtkTreeIter)` too. And the rest of the iteration functions (`start`, `done`, `next`, `prev`, also `+`, `-`, `skip`, `up`, `down`, and the `is_...` methods, maybe even `contains` and `isempty`). See implementation of `GtkTextIter` for ideas (although that was an opaque datatype, whereas this, fortunately, is not)