We should avoid recursion, as it makes reasoning about stack growth
harder. This tool shows (currently) 4 (non-tail) recursive functions, at
least 2 of which are easy to fix.
This allows Tox to contain additional data on top of Messenger, making
Messenger not necessarily the most top-level object. E.g. groups are
built on Messenger and currently awkwardly void-pointered into it to
pretend there is no cyclic dependency.
