The Duration record no longer exists in Temporal. Implement it according
to the DurationFormat spec to prepare for its removal from our Temporal
implementation.
We also implement the DurationSign AO here as well, as the Temporal
implementation will now require a Temporal.Duration JS object.
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
