This improves the implementation of the generated parent/child
relationship by adding a new `all_children` field to `ql.Class` which
lists all children (both direct and inherited) of a class, carefully
avoiding duplicating children in case of diamond inheritance. This:
* simplifies the generated code,
* avoid children ambiguities in case of diamond inheritance.

This only comes with some changes in the order of children in the
generated tests (we were previously sorting bases alphabetically there).
For the rest this should be a non-functional change.

…mission-trait