This is in preparation for allowing the run-time / items to clone VRc's
of the component.
ComponentVTable functions like visit_children_item contine to take a
ComponentRefPin as "self" parameter type, as a VRc would not be
supported by rust right now. That means the implementation then uses
self_weak to obtain a strong self-reference.
When converting a Weak<VT, T> into Weak<VT, Dyn>, we were ignoring that
while T inside VRcInner may be at a certain offset to satisfy alignment,
the empty Dyn field may not and thus be at a different offset.
On wasm32 for example the compiler generated component struct has a
16-byte alignment, while the Dyn has none. So converting the Weak<VT,
Dyn> to a VRc<VT, Dyn> and then calling borrow() would use the wrong
instance pointer.
To fix this, this patch introduces an extra offset field that's
preserved during the casting and that's used for borrow() as well as
drop().
Removed the drop and create from the ComponentVTable:
since we are not using VBox<ComponentVTable>, this simplifies a bit
the code of the interpreter and everything else.
But there is still a lot of changes everywhere to support that the Component
is pinned.
This is just for the component. Which would be required if later we want
to access the properties as Pin<Property<_>>. But we have not yet ability
to do projections
