`advance_to` not implemented for `roaring::treemap::Iter`

[sdd]

I have a use case where I need the advance_to method, which is present on RoaringBitmap's Iter, but I want to use it from a RoaringTreemap. This doesn't seem to exist - I'm happy to contribute an implementation, unless there a reason why this is not implemented already that I'm unaware of. Would you be open to this?

Additionally, I looked into implementing a struct outside of this library that would provide this functionality, but it's not possible to implement this easily because roaring::treemap::iter::BitmapIter is not publicly exported. Is this just an oversight? It's not currently possible to store the result of the public RoaringTreemap::bitmaps method in a struct because of this.