Implement logic to derive blinded keys and subcredentials for onion services

To access an onion service, we need to know its blinded "key of the day", and a "subcredential" string that's used to derive other keys about it. See rend-spec-v3.txt section 2.1.

Key blinding is already implemented in tor-llcrypto, so that's nice. But we'll need some basic glue code and types to implement the key functionality here, and not just have a bunch of exposed [u8;32].

This is a fairly early prerequisite for other Onion Services work.