kernel: _ClassInfo nodes own a lot of empty Sets

[rakudrama]

In my large ACX app compile scenario, 51288 _ClassInfo nodes retain 68.9MB, or about 1400 bytes on average.
146730 of the 153864 directExtenders, directMixers and directImplementers Sets are empty, i.e. 95%.

This is expected because leaf classes dominate in the application - all enum classes, protobuf messages classes and ACX template classes are leaf classes.

If these Sets were created lazily just before adding, it would save ~35MB, or half of the current heap retained by the _ClassInfo nodes (genericSuperType and genericSuperTypes are created this way but are actually less sparse in the app).

[rakudrama]

It is possible that a fix for #26081 would reduce the empty-Set overhead from 100% to 25%.