lisp/emacs-lisp/fcr.el: Signal errors for invalid code

lisp/emacs-lisp/fcr.el: Signal errors for invalid code

* test/lisp/emacs-lisp/fcr-tests.el (fcr-tests): Remove left-over
debugging messages.
(fcr-tests--limits): New test.

* lisp/emacs-lisp/fcr.el (fcr-defstruct): Fill the `index-table` and
signal an error in case of duplicate slot names.
(fcr-lambda): Change use of `fcr--fix-type` so `cconv-convert` can use
it to detect store-converted slots.  Tweak generated code to avoid
a warning.
(fcr--fix-type): Adjust accordingly.

* lisp/emacs-lisp/cconv.el (cconv-convert): Signal an error if we
store-convert a FCR slot.