Speed up DynamicJaxprTracer.__init__.
* Don't create MutableQDDs if we don't need them. Make mutable_qdd optional to save an allocation most of the time.
* Use Tracer.__init__() rather than super().__init__(). This is slightly faster in a profile.
* Add __slots__ on JaxprStackFrame to avoid some dictionary lookups.
PiperOrigin-RevId: 917955593
