Key blinding in onionbalance v3
Need to implement key blinding for v3 onions in onionbalance (or stem).
There are already some tests in little-t-tor about it but they seem to be using a custom ed25519 implementation, whereas stem is using hazmat: https://gitweb.torproject.org/tor.git/tree/src/test/ed25519_exts_ref.py#n34
Need to decide whether I should do the same for hazmat, or adopt the custom ed25519 implementation. Most probably the former.