This is how the spec suggests implementing this; we need to be slightly
more verbose as our PromiseCapability implementation cannot hold
arbitrary JS values.
Unfortunately it makes the error message slightly more ambiguous as we
no longer expose the non-function value to the outer scope (we could!),
but at least we don't UAF the stack allocated values anymore :^)
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.
