It's easier to reason about the activity.js code if we just
make it clear that presence_info is a global singleton. Going
forward, functions that use that data will explicitly refer to
exports.presence_info.
This change makes some tests slightly more awkward to set up, but
with the actual code you now don't have to question whether there
are multiple copies of presence_info to maintain.
The diff for compare_function is a bit confusing to read, but it's
basically just de-dented since we don't need to parameterize the
compare function any more.
a246d2dcb3