These are slightly unfortunate as we're crossing the library boundary,
but there's precedent with Object::is_dom_node(), and these are just
knocking down a few more items that were showing up in profiles.
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
