The one current caller of this function always defers microtask
checkpoints before calling wait_for_all, ensuring that the promise
accept/reject handlers will always be called later in the Web event loop
processing. We need to store all the state for the closures in a heap
allocated object with HeapFunctions to keep it around while there are
still promises to resolve.
This makes Fetch rely less on using main_thread_vm().current_realm(),
which relies on the dummy execution context if no JavaScript is
currently running.
A struct with three raw pointers to other GC'd types is a pretty big
liability, let's just turn this into a Cell itself.
This comes with the additional benefit of being able to capture it in
a lambda effortlessly, without having to create handles for individual
members.
