Features for 0.1.0

[casey]

There are a few features I'd like to add before 0.1.0:

• Check keychain directory and private key permissions on Windows
• Add unit tests and test vectors for hashes and signatures
• Add version field to manifest
• Add version field to metadata
• Consider including time by default when signing
• Allow specifying a different time when signing
• Consider switching bech32m version character to q?
• Consider making metadata errors a warning by default, and allowing opting into an error with a lint
• Drop the .json from filepack.json so it can be changed to some other format eventually?
• Consider making manifest CBOR