SQLite

I dunno if this is WIP or what.  It's clearly a cleaner approach, but this in itself doesn't provide any new functionality at all, and merging means rebuilding all the ultragrep indexes out there.  So I dunno.  maybe this is RFC?  I also am not sure that this will be the final table format.

@vanchi-zendesk 
just to reduce confusion, simply call this offset (or raw_offset everywhere in this file)