It is currently implemented as a member of CyclicModule. However, as the
spec indicates, this must be invokable with non-CyclicModule modules. In
several of the call sites, we are blindly casting to a CyclicModule;
this will fail for e.g. JSON modules.
Resulting in a massive rename across almost everywhere! Alongside the
namespace change, we now have the following names:
* JS::NonnullGCPtr -> GC::Ref
* JS::GCPtr -> GC::Ptr
* JS::HeapFunction -> GC::Function
* JS::CellImpl -> GC::Cell
* JS::Handle -> GC::Root
