These are a bit complex internally, but the API they present is
pretty simple. I've left some discussion of points where the design
isn't totally fleshed out, and where we need to look harder at the
spec.
Part of #716.
This module has types and operations needed in multiple places
for an onion service implementation. There are a bunch of
TODO hs-crypto comments that we'll need to fill in.
Nick Mathewson