hs descriptor decoder

We need something to decode hs descriptors. This can involve the use of private key material!

Part of #683 (closed). See the notes there.

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Remaining tasks for MVP:

• Check whether the C implementation enforces the absence of extraneous bytes at the end of the linkspecs.
• Decide whether we need to validate the semi-bogus certs in the inner document, check their timestamps, and enforce all the spec's requirements on them.
• Decide whether the expiration time on the certificate in the outer document should apply to the descriptor as a whole.