TorClient::dirmgr() and circmgr() should return &Arc.

Right now these functions return an Arc<...>. But on !347 (2f2ba8db, comment 2782761) , @Diziet says:

I would usually have a method like this return &Arc, so that it doesn't clone. (Call sites that need a clone can make one of their own (albeit with some syntatic vinegar, unless we take !352 (merged))) But I don't know whether your call sites all (or mostly) need owned data.

I think it's a good change to make, so that callers only make this change when needed.

Since these methods are under experimental-api, this isn't a semver-breaking change.