Skip safety rails for widgets with a large number of children

[PoignardAzur]

Safety rails are a debug feature that panic early if they detect that a widget was implement incorrectly. They only run with debug_assertions, and overall documentation is careful to never guarantee that they'll run. They're here as an additional help for users.

When we're running with debug assertions, we assume that user doesn't care too much about performance and a small overhead is fine. However, safety rails can add large (think O(N²)) overhead to passes when widgets have a very large number of children.

We should add an automatic escape hatch for cases where children_ids is large, so we can skip children in those cases. We can assume that if MyCustomWidget has a bug in child handling, the will probably show up with a small number of children too.