hsservice: Initial data structures and APIs
This branch merges structures and APIs from our design notes into the tor-hsservice crate.
Closes #970 (closed). Closes #971 (closed). Closes #972 (closed).
@Diziet @gabi-250 Probably you should both have a look at this, though since it's a draft I think it's okay to merge while we still have open discussions.
Edited by Nick Mathewson