Add CCBV.{fold, fold_true}?

[favonia]

I wonder if CCBV.fold_true could earn its place in the library? I found it quite convenient when implementing search algorithms, but perhaps others would know other interesting uses. A generic folding seems to fit the library, and is present in the bitv library.

EDIT: after some major refactoring, the short-circuiting CCBV.for_all_true seems to be less useful.

[c-cube]

Sounds useful, yes :). There is already the iter equivalent of it.