While we don't yet have a working `using` implementation with our byte
code, we can still keep our DisposableStack implementation up to date.
The changes brought in here are all editorial, and set us up to start
an AsyncDisposableStack implementation.
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
