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
While this is used in the implementation of Runtime objects itself, Heap
seems like a more appropriate home. This will also help in factoring out
the GC implementation into it's own library as the heap explicitly has
knowledge of WeakContainer.