We were unnecessarily discarding the shadow trees of various elements
when they were removed or detached from the DOM.
This especially caused a *lot* of churn when creating input elements via
setting .innerHTML on something. We ended up building each input
element's shadow tree 3 times instead of 1.
The original issue that we were trying to solve by discarding shadow
trees appears to have been solved elsewhere, and nothing else seems to
break by just allowing them to remain in place.
1.05x speedup on Speedometer's TodoMVC-jQuery.
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
Now that we have RTTI in userspace, we can do away with all this manual
hackery and use dynamic_cast.
We keep the is<T> and downcast<T> helpers since they still provide good
readability improvements. Note that unlike dynamic_cast<T>, downcast<T>
does not fail in a recoverable way, but will assert if the object being
casted is not a T.
