* lisp/find-file.el: Use lexical-binding

* lisp/find-file.el: Use lexical-binding

Remove unused `:group` args.  Prefer #' to quote function.

(ff-special-constructs, ff-find-the-other-file, ff-get-file-name)
(ff-list-replace-env-vars, ff-cc-hh-converter): Use `match-string`.
(modula2-other-file-alist): Tighten regexps.
(ff-get-other-file, ff-find-other-file): Use dynamic scoping.
(ff-find-the-other-file): Minor simplification.
(ff-other-file-name): Delete unused function.
(ff-string-match): Don't let-bind `case-fold-search` if not needed.
(ff-basename): Make it an obsolete alias for `file-name-nondirectory`.
(ff-switch-file): Minor simplification.
(ff-list-replace-env-vars): Use [:alnum:].
(ff-upcase-p): Use [:upper:]
(ff-cc-hh-converter): Use [:upper:] and [:lower:].