This patch synchronizes changes from whatwg/fetch#1569 and
resolves a related FIXME: "Refactor this to the new version of the
spec introduced with whatwg/fetch@464326e.”
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