Solve pending TODO HS issues with TorAddrError::BadOnion

Currently, our comments indicate that we think that TorAddrError::BadOnion should contain a HsIdParseError, but it doesn't, because HsIdParseError is not PartialEq.

Another complicating issue is that TorAddrError is a public part of the arti_client API: it isn't opaque like arti_client::Error is. That means we need to be really careful what we add to it.

I am calling this a Blocker since it's a top-level API issue.

So, here's my proposed "least-bad, not very breaking" solution:

• Include HsIdParseError in TorAddrError::BadOnion.
• Refactor HsIdParseError to not expose tor_bytes::Error or data_encoding::EncodeError. This is the "bad" part.
• Add comments to HsIdParseError and TorAddrError reminding the developer that these are part of our arti_client API and we need to be careful what we put in them.

@gabi-250 @Diziet any objections or counterproposals?