Clean up hs_ntor.rs, add test vectors generated by C tor, and fix some bugs
This branch cleans up some interface cruft in hs_ntor.rs, removes some duplicated code, and adds a test case that I generated using C tor and a bunch of extra log statements. While making the test pass, I found a couple of bugs in the way that we generated one of the MACs. (This is why round-trip tests are not enough!)
We might need to make further API cleanups here down the road.
Closes #865 (closed).