Implement local encryption

[gferon]

Implement encryption of the values stored in the sled::Db which is in theory the last missing feature in presage 🥳

This breaks again the compatibility of previous sled stores (sorry 😢) since I also took the opportunity to refactor the code and introduce this change with more removed code than added (!)

EDIT: will not merge this before I come up with some sort of schema migration system (which shouldn't be too hard, but will take a bit of extra time)

[Schmiddiii]

Overall this looks good to me, but I have some issues I am currently unsure about (and don't know if they even apply):

• The tree-names are not encrypted, it is therefore possible to extract all threads (including contacted UUIDs and GroupMasterKeys) from an encrypted DB
• Messages and Contacts are currently stored unencrypted (as far as I understand the code, only self.[get,insert] encrypts, but these are directly executed on the self.db)
• Keys are not encrypted. I am not sure that is a huge security-risk, but that should be thought about.
• There is a big warning sign in crates.io but I think this should be ok.

Maybe my understanding of the code is also wrong.

[gferon]

There is a big warning sign in crates.io but I think this should be ok.

Just to address this comment, this is exactly used as intended (and is also used in a similar fashion in the matrix-sdk). presage is not a final application but also a building block.