Cache Sealing and Opening keys in secrets

[djc]

The crypto module currently has a PacketKey type that reconstructs the relevant SealingKey and OpeningKey for every seal or open operation. There should be a smarter way to structure the types in the crypto module so that a Secret has direct access to long-lived Key instances.

[djc]

@stammw is this something you want to try next? (The description isn't quite valid anymore as some of the type names are different from when I wrote this up, but the gist of it still holds.)

[stammw]

Sure ! I'll see what I can come up with !

[djc]

Cool. Don't hesitate to ask if you have questions!

[stammw]

@djc I believe this issue can be closed.

[djc]

Yup! Thanks for working on it.