* lisp/mail/footnote.el: Use dolist and hoist regexps out of loops

* lisp/mail/footnote.el: Use dolist and hoist regexps out of loops

(footnote--refresh-footnotes): Use pcase-dolist; compute regexp once
outside of the loops.  Use less confusing `literal` arg to `replace-match`
and specify `fixedcase` since footnote--index-to-string already chose
the proper case for us.
(footnote--renumber): Use dolist; compute regexp once
outside of the loops; shortcircuit when number is unchanged.
(footnote--text-under-cursor): Rewrite.
(footnote--make-hole): Use dolist.
(footnote-add-footnote): CSE.
(footnote-delete-footnote): Use dolist; compute regexp once
outside of the loop.
(footnote-delete-footnote): Don't renumber if there's no footnote left.
(footnote-renumber-footnotes): Use dolist.