Enable memory-only key persistence

[paragonie-scott]

For version 2 and 3:

Update KeyFactory to support loading/saving keys to an encoded/serialized string format. We'll probably use the exact same structure as saving to a file.

In version 2: work with strings.
In version 3: work with HiddenString objects.

This will be released as 2.2.0 and 3.1.0, respectively.