Channels need to be removed from the global array/map when destroyed.
Note that this exposes an infinite loop in map_remove, which will
be fixed later.
Note also that Channel's are retained if they have any messages in
them, to prevent releasing a channel while it has pending messages.
- Ref struct only stores refcount now and is more general.
- Proxy stores a hash of its type name instead of an enum.
- Variants store additional information instead of using a vtable.
- Remove the concept of superclasses from the API.
- Clean up some miscellaneous includes.