Because of how name builders used to lazy gather namespaces on the first
requested name, the objects were self mutating. As such only non-const
references could be used to pass them around.
Since that is no longer the case, this restores most of lost const
qualifiers.
This reworks NameBuilder to ease the transition to the system where
the model will be guaranteed to never contain any name collisions, both
between user-specified names and the automatic ones.
Ivan Krysak