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.
The hard part of parsing them in import statements and calls was already
done so this is just removing some check which threw before on
assertions. And filtering the assertions based on the result of a new
host hook.
