Use slab instead of the deprecated vec_arena, both of which appear to
use the same Vec based storage of an enum that's either the entry or an
index to the next free slot.
* sixtyfps_timer_start needs to *take* the timer id out of the Rust
timer to avoid that the subsequent drop stops the timer again
* For the Qt event loop, call `timer_event()` once before entering
QCoreApplication::exec(), to schedule any timers that were started
beforehand.
* Added a way to quit the event loop gently, in order to use that
from the C++ unit test.
In the nightly it appears that `no_mangle` is now considered "unsafe",
so we need to allow that in the ffi modules. For the layout code this
patch also creates that ffi module with prefixed function names, like in
the other modules and only allows unsafe in there.
Don't require the caller to Box the closure. With the assumption that
the majority of callers *want* the closure to be boxed (i.e. it's not
already boxed), the API becomes easier to use.
