Grothendieck Construction

We want to have the following as subdirectories/modules, not at top level. Unfortunately, namespacing in Coq is kind-of broken (see, e.g., https://coq.inria.fr/bugs/show_bug.cgi?id=3676), so we don't get the ability to rename subfolders by Including into other modules.

of a functor to Set

of a pseudofunctor to Cat

of a functor to Cat