Encode and decode onion service descriptors

Onion service descriptors use the same metaformat as other Tor network documents, and should probably be encoded/decoded behind an optional feature in tor-netdoc. The format is documented in rend-spec-v3.txt sections 2.4 and 2.5.

We should separate encryption/decryption concerns from parsing/encoding concerns: this will probably mean a two-stage process for encoding/decoding.

To get example desriptors for testing, we can run a chutney network that uses onion services, with instrumentation in the C tor to cause it to dump the descriptors it generates.

Subtasks (may not be an exhaustive list):

• #743 (closed) netdoc metaformat encoder
• #744 (closed) hs descriptor decoder
• #745 (closed) hs descriptor encoder