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.
LibWeb keeps growing and the Web namespace is filling up fast.
Let's put DOM stuff into Web::DOM, just like we already started doing
with SVG stuff in Web::SVG.
