comp-cstr.el: The type hierarchy is a DAG, not a tree

comp-cstr.el: The type hierarchy is a DAG, not a tree

Adjust the type operations to account for the fact that types can have
several parents.

* lisp/emacs-lisp/comp-cstr.el (comp--cl-class-hierarchy):
Use `cl--class-allparents`.  Add FIXME.
(comp--direct-supertype): Declare obsolete.
(comp--direct-supertypes): New function.
(comp--normalize-typeset0): Rewrite to use `comp--direct-supertypes`;
adjust to account for the DAG structure; use `cl-set-difference`.
(comp--direct-subtypes): Rewrite.
(comp--intersection): New function.
(comp-supertypes): Rewrite and change return type.
(comp-subtype-p): Simplify.
(comp-union-typesets): Use `comp-supertypes` instead of iterating over
`comp-cstr-ctxt-typeof-types`.
* lisp/emacs-lisp/comp.el (comp--native-compile): Don't catch
errors if we're debugging.
* test/lisp/emacs-lisp/comp-cstr-tests.el: Adjust tests.

* lisp/emacs-lisp/cl-macs.el (cl-defstruct): Fix mishap when we
evaluate (cl-defstruct cl-structure-object ..) during the compilation
of `cl-preloaded.el`.
* lisp/emacs-lisp/cl-preloaded.el: Add corresponding assertion.