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.
Most system headers contain functions (e.g. `memcpy` in `string.h`)
which aren't needed in our own header files. For the most part, our own
headers should only include types needed to declare our own types and
functions. We now enforce this so we think twice about which headers we
really need in the .h files.
Also added a valgrind build to run it on every pull request. I've had to
disable a few tests because valgrind makes those run infinitely slowly,
consistently timing them out.
It's nice we are able to compile with `tcc`. Let's not break that.
CompCert is also neat, but its interpreter mode doesn't work on tox, so
we only use the compiler.
