This is an SQLITE3 backend for ERIS, implementing
https://eris.codeberg.page/eer/sqlite.xml

Due to the foreign dependency and the fact that it may not be suitable for major
usage, it is not included in the main repo. I don't recommend using this backend for writing for two reasons:

1. Since sqlite is a relational database, not a hash-table, fetching each
reference takes O(logn) time (with indices). This might hurt compared to a
hash-table O(1) in cases where there's millions of blocks.

2. Unless your filesystem sucks at handling lots of small files, there is little
benefit over the simple file-based backend. This might be a major motivation for
using this backend in FAT32 (65536 max files in a directory ~ max 2GB of ERIS
data) or NTFS (lots of small files kill it).