* lisp/emacs-lisp/bindat.el: Clarify when field labels are optional

* lisp/emacs-lisp/bindat.el: Clarify when field labels are optional

The fixes the doc and the Edebug spec, as well as a subtle issue in
the code where a field whose name is (eval 'fill) was mistakenly
considered as an anonymous field of type `fill`.

(bindat--unpack-item, bindat--unpack-group, bindat--length-group)
(bindat--pack-item, bindat--pack-group): Use dotimes, dolist, and pcase.
(bindat--item-aux): New edebug elem.
(bindat-item): Use it to fix the handling of optional fields.
(bindat-format-vector): Use `mapconcat`.