While PendingPullIntos are typically visted by their controller there
were some cases that we were removing those references from the
controller and storing them in a SinglyLinkedList on the stack which
is not safe.
Instead, make PendingPullInto a GC::Cell type, which also allows us
to remove an awkward copy of the struct where the underlying reference
was previously being destroyed.
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
