[WIP] aead: add PrefixNonceAead

[tarcieri]

This is another attempt at #1666 (see also #1818).

Where #1818 tried to add some additional trait methods, this instead opts for a generic newtype which provides type safety around the use of prefix messages (i.e. avoids confusion around accidentally mixing up prefixed and non-prefixed messages by calling the wrong methods) and in general keeps the method space unambiguous so we don't need to worry about qualifying every method name with something to indicate it's working on nonce-prefixed messages.

TODO: tests, docs, but I'm posting this early to get feedback on the general direction

[tarcieri]

I suppose this is always assuming a postfix nonce, I will need to further compose it to abstract over prefix vs postfix nonces