Leekspin descriptor signatures cannot be verified by Stem

The signatures on descriptor documents generated by Leekspin (currently on version 0.2.1), in leekspin.crypto.signDescriptorDocument(), cannot currently be verified by Stem.

Stem uses this code to successfully verify signatures created by Tor. There is currently some confusion in the spec (#13042 (moved)) over the ordering and versions of encodings applied to the signing-key and the router-signature. Until #13042 (moved) is made clearer, the best way we have to fix this is to do what Stem does in reverse.

There may be some problems here with Python dependencies (the available, packaged, implementations of RSA, PKCS!#1, and ASN.1 aren't all that great, as noted in #5810 (closed)). I don't care what dependencies we add to get this to work; it's causing BridgeDB's new Stem-based parsers (#9380 (moved)) to choke during test runs on Leekspin's fake bridge descriptors.