Modernize, revise, and test hs_ntor.rs

Our HS-NTor implementation (a.k.a NTOR-WITH-EXTRA-DATA from rend-spec-v3.txt) was originally written as a learning exercise. Now it's a bit out of data and needs modernization, documentation, and more tests.

I am hoping that this won't take too long, but I am budgeting extra time because of the current lack of any testing besides "can this round-trip with itself".