Rename hs_index and hsdir_index to hs_{service,relay}_index
These new names are the ones used in arti's hsdir_ring.rs and make a lot more sense than calling one of them the "directory" index and the other just the "index".
In C Tor these are calculated by functions called
- hs_build_hs_index
- hs_build_hsdir_index
That might be a reason not to accept this change. Or it might be a reason to change the C Tor code.
If we don't change the names in the spec the Arti function names should change.