Use RoaringBitmaps for indexing in kvschema

[burdiyan]

I was really glad to see that I'm not the only one thinking about a nicer abstraction layer for key-value stores like Badger. The package kvschema looks really nice! One thing I'd like to suggest is to use roaring bitmaps for indexing instead of storing uint64 as byte slices.

Would you accept a patch for this, or I better create my own code generator in a similar vein for my own needs?

[jtacoma]

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.