Add convenience functions for generating Emacs Lisp files

Add convenience functions for generating Emacs Lisp files

* lisp/url/url-cookie.el (url-cookie-write-file):
* lisp/international/titdic-cnv.el (tit-process-header):
* lisp/international/ja-dic-cnv.el (skkdic-convert):
* lisp/international/emoji.el (emoji--generate-file):
* lisp/emacs-lisp/loaddefs-gen.el (loaddefs-generate--rubric)

* admin/unidata/unidata-gen.el (unidata-gen-file)
(unidata-gen-charprop): Use the new functions.

* lisp/emacs-lisp/generate-file.el: New file to provide
convenience functions for generated files.  It's not always
trivial to know which parts of the trailer that has to be
obfuscated to avoid not getting byte-compiled etc, and some parts
of the headers/trailers are usually forgotten when hand-coding
these.