Formalize the cumulative hierarchy V (most of §10.5 of the book)

This is joint work with Bas Spitters.
It formalizes exercise 10.11 (the "pushout" definition of V), where we had to slightly modify the induction principle given in the book in order to make it provable (see V_rect').
Then most of section 10.5 is proven (up to theorem 10.5.8).
Is there a reason to write `Definition ... := fix` rather than `Fixpoint ...`?