refactor!: restructure project for better DX and UX

[sseemayer]

This mega-PR is not intended to be merged, but to inform and get feedback from everyone. I want to explore a new design for this crate that hopefully is cleaner, plus easier to use, understand, and extend.

@louib @varjolintu @droidmonkey I would appreciate suggestions on how to break down this PR into several smaller PRs that can be reviewed individually.

• replace the previous design keepass::db using nested Group structs with a flat design that stores Groups, Entrys, BinaryAttachments in the Database
  • Newtypes (e.g. GroupId(Uuid)) to refer to the internal objects
  • Accessor types (e.g. GroupRef and GroupMut) to safely and conveniently expose database state, while enforcing internal consistency.
• replace keepass::error with module-local, more granular error enums that #[derive(thiserror::Error)]
• rewrite xml_db using quick-xml with serde (de)serialization of XML instead of xml-rs event-based parsing
• reimplement KDB loading
• reimplement KDBX3 loading
• reimplement KDBX4 loading
• reimplement History parsing
• reimplement KDBX4 writing
• reimplement merging
• implement immutable iterators for groups and entries
• implement mutable iterators for groups and entries
• ensure the same or better test coverage than before

BREAKING CHANGE: Different database types and how they are accessed. Removes the Node, NodeRef, etc concepts.
BREAKING CHANGE: Completely replaced error types to make them more granular

[droidmonkey]

FWIW, I had copilot agent analyze and propose a solution to transactional edits to the database. It also proposed a JSON & API based method for these transactions. This was a thought project on how we could implement immutable database/group/entry sharing in KeePassXC and move all editing into a centralized place. droidmonkey/keepassxc#6

[sseemayer]

FWIW, I had copilot agent analyze and propose a solution to transactional edits to the database. It also proposed a JSON & API based method for these transactions. This was a thought project on how we could implement immutable database/group/entry sharing in KeePassXC and move all editing into a centralized place. droidmonkey/keepassxc#6

Controlled mutable access via GroupMut / EntryMut / etc is one of the benefits of this new design. I was imagining some sort of GroupTracked that is GroupMut, but tracks what changes it performs and persists them to the history upon drop.