The stack reads its own descriptors to figure out which endpoints
are used in which alt settings, and enables/disables them as needed.
The ControlHandler has a callback so it can get notified of alternate
setting changes, which is purely informative (it doesn't have to do anything).
The same API call allocates interfaces/endpoints/etc and writes their descriptors.
This means less API calls, and less possibility to screw things up.
DescriptorWriter is now private.
