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).
Perhaps it would be better to define it in terms of `is0trunc_V`, outside of the module, with a comment on the inside saying that we don't need to axiomatize the computation rule because it's automatic, but it's defined outside for regularity?