[annotation] Add begin/finish annotations for GAsyncResult
Submitted by Travis Reitter
Some languages (like Vala) support native syntax for asynchronous functions.
Vala's GLib/C parser has a heuristic that treats pairs of functions with signatures
void $(foo)_async (..., GAsyncResultCallback cb, ...); $(type) $(foo)_finish (..., GAsyncResult *res, ...);
as a native async function returning type $(type).
In order for Vala's (and other languages') GIR parser to generate the same binding, without having to hard-code this heuristic, g-ir-scanner/compiler will need to implement it and adjust GIR output accordingly.