`SlottableMixin::assigned_slot()` was using `as<DOM::Node>(*this)` to
get a `Node` reference. Since `SlottableMixin` has no inheritance
relationship with `Node`, `as_if<>` can't use `static_cast` and falls
through to `dynamic_cast`, which is expensive. Replace this with a
virtual `slottable_as_node()` accessor overridden in `Element` and
`Text`.
This showed up as hot in profiles when loading the GC heap explorer
page.
Returning a Vector of Slottable is not very nice here, but this
matches find_slottable (which this calls), and as far as I can
tell this is technically 'safe' at the moment in the way in
which it is / will be called.
It's also not great that like find_slottable it takes a non-const
ref, but changing that causes a bunch of other fallout.
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
