Use `sled` for persistent secret storage, `linux-keyutils` for caching

It turns out Linux keys cannot (or should not) be used for long-term, persistent secret storage. This will introduce a sled database for storing secrets on disk in the stash directory. Linux keys have been repurposed for secret caching to avoid disk I/O whenever possible.

• Introduces Secret struct and associated methods for generating or storing encryption secrets
• Adds the db field to the Stash struct
• Stores sled database in stash directory at .db
• Adds a hidden file .secret to store key and nonce for .db (not encrypting it, though, for now)
• Preferentially checks for and uses Linux keys over db entries
• Adds error checking for existing files to prevent data corruption on bad encrypts/decrypts