Instead of synchronously handling events as they happen in
`tox_iterate`, this first collects all events in a structure and then
lets the client process them. This allows clients to process events in
parallel, since the data structure returned is mostly immutable.
This also makes toxcore compatible with languages that don't (easily)
support callbacks from C into the non-C language.
If we remove the callbacks, this allows us to add fields to the events
without breaking the API.
Currently: 1) libsodium and 2) nacl.
Note that the "nacl" variant is actually libsodium. We just want to make
sure the static analysers see the `VANILLA_NACL` code paths.
This check puts all of our code in a C++ anonymous namespace, which is
effectively making all functions `static`. This allows the compiler to
determine that a function is unused, so we can delete it.
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.
