Add module_info.get_all

[eric-wieser]

The motivation here is to teach doc-gen about empty default.lean files.

[eric-wieser]

What's the easiest way to convert an std::vector<T> into a vm_obj holding a list? I was hoping to_obj had an overload for that.

[gebner]

I think vm_list.h is everything we have. Copy&pasted from to_obj(buffer<expr> const &):

    vm_obj obj = mk_vm_nil();
    for (unsigned i = ls.size(); i > 0; i--)
        obj = mk_vm_cons(to_obj(ls[i - 1]), obj);
    return obj;