This adds a mechanism for running generic operations at specific points
of the frame execution. This can be used by any part of the code an not
just Lua since it simply executes std::functions. It also adds an
"execution context" concept that specifies in which situation an
operation should be executed.
This is then used by the Lua Coroutines code for resuming coroutines at
specific points instead of always when the awaited promise resolves.
