Use RoaringBitmaps for indexing in kvschema

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?