Expose a method on built-in iterators that allows retrieving the next
iteration result without allocating a JS::Object. This change is a
preparation for optimizing for..of and for..in loops.
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
