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.
This is mostly to get the grunt work of the way. This is split up into
multiple commits to hopefully make it more manageable to review.
Note that these are not full implementations, and the bindings mostly
get the low hanging fruit.
Also implements some attributes that I kept out because they had
dashes in them. Therefore, this closes#2905.
