This is a normative change in the ECMA-262 spec. See:
de62e8d
This did not actually seem to affect our implementation as we were not
using generators here to begin with. So this patch is basically just
adding spec comments.
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
