Improve wallet login by introducing a user password

[mmsinclair]

Currently it requires the mnemonic every time, it would be nice if the user had an option to save the generated key and protect it with a password

[mmsinclair]

@jstuczyn I was thinking you could use https://docs.rs/rust-crypto/0.2.36/crypto/pbkdf2/fn.pbkdf2.html to generate a symmetrical key from a password. An RSA encryption key would be randomly created from an IV that would be encrypted with the pbkdf2 key and written as a fixed length byte buffer to disk. The RSA key would encrypt the mnemonic and that would be written after the encrypted IV.

It would be worth wrapping it in JSON (you can split out the encrypted IV and payload) and base64 the cyphertext.

{ format_version: 1, iv: ..., payload: ... }

[mmsinclair]

@jstuczyn this might also be useful: https://docs.rs/tauri/1.0.0-beta.8/tauri/api/path/fn.home_dir.html