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
Until now we would simply apply stylesheets in the order they finished
loading. This patch adds a StyleSheetList object that hangs off of each
Document and contains all the style sheets in document order.
There's still a lot of work to do for a proper cascade, but at least
this makes us consistently wrong every time. :^)
