Corresponds to dba6e7b6c2
and 4c0401186c
The spec algorithms now use "the relevant global object's associated
document", so remove the concept of the History object itself having an
associated document. The spec has also combined the implementations for
forward/back/go so I've matched that too.
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
