SpiderMonkey 52 includes native promises, so we can remove the embedded
JS implementation of promises. Queueing up and resolving of promises is
now done inside SpiderMonkey itself, and we must implement some
SpiderMonkey extension points in order to get this to integrate with our
GLib main loops.

The implementation is adapted from what SpiderMonkey does internally in
its js::RunJobs() function; see the code in jscntxt.cpp.

https://bugzilla.gnome.org/show_bug.cgi?id=784713