Switch to redb backend

[GregoryConrad]

Having the LMDB backend from milli is the root of a lot of problems (#10, #101, #227); we need a redb polyfill over in heed.

Unfortunately, I won't be able to get around to this for quite some time. If you know Rust, or want to learn some, I'd really appreciate some help here. I'd be more than happy to help guide you on what to do to get this landed.

[frederikstonge]

Is it planned soon? Also, from what I've seen, redb is not as powerful in terms of search engine, will affect the package features?

[GregoryConrad]

Not planned at all, unless someone from the community does it.

The idea is that redb would be a replacement for LMDB over in milli—so same functionality, just different backing store.