This is very clearly a very dangerous API to have, and was causing
a crash on Linux as a result of a stack use-after-free when visiting
https://www.index.hr/.
Fixes#3901
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