HB.instance with a better complexity

[gares]

EDIT by @CohenCyril :
Right now instance declaration iterates over all classes, tries to find an instance, and instantiates the class / structure or aborts silently if it fails. This would be better achieved using the following algorithm instead:

1. compute the set T of classes that are potential targets to instanciate
2. compute the set E of already existing instances
3. compute the set difference T \ E, topological sort it and iterate on that set instead.

CC @gares @ThomasPortet