Use RoaringBitmaps for indexing in kvschema about note-maps HOT 1 OPEN

Alexandr, I'd be happy to get a pull request! I hadn't heard for roaring bitmaps before, it looks like a good fit.

We're very pre-1.0 here so don't worry too much about backward-incompatible changes: if there are any, I'll just increment the minor version number.

FYI, I'm not sure I'll be using kvschema in Note Maps despite all the time I put into it. I still like the idea, and even the implementation, but I'm looking at higher-level abstractions these days. It's all still kind of speculative, we'll see what happens.

from note-maps.