next version

ended up being a significant rewrite oops! - drop sqlite, pick up fjall v3 for some speeeeeeed (and code simplification and easier build requirements and) - no more `Processable` trait, process functions are just `Vec<u8> -> Vec<u8>` now (bring your own ser/de). there's a potential small cost here where processors need to now actually _go through_ serialization even for in-memory car walking, but i think zero-copy approaches (eg. rkyv) are low-cost enough - custom deserialize for MST nodes that does as much depth calculation and rkey validation as possible in-line. (not clear if it actually made anything _faster_) - check MST depth at _every_ node properly (previously it could do some walking before being able to check and included some assumptions) - check MST for empty leaf nodes (which not allowed) - shave 0.6 nanoseconds (really) from MST depth calculation (don't ask) i guess i should put this in a changelog