arti-client: Make dirmgr() and circmgr() return &Arc<..>
Previously they returned an Arc, which wasn't necessary unless the client actually wanted a new Arc.
This would be an API break, except that these functions are marked 'experimental-api', so semver does not apply; nonetheless I've noted the break in semver_status.md, just in case we care.
Closes #369 (closed)