* lisp/filesets.el: Use lexical-binding

* lisp/filesets.el: Use lexical-binding

Remove redundant `:group` args.  Require cl-lib and seq.
Fix various O(n²) bug and flag a few remaining ones.

(filesets-external-viewers): Simplify regexps.  Use \' instead of $.
Remove useless :constraint-flag properties.
(filesets-convert-path-list): η-reduce.
(filesets-eviewer-constraint-p): Mark :constraint-flag as obsolete.
(filesets-spawn-external-viewer): Can't use `run-hooks` on
lexical variable.
(filesets-filter-list): Fix O(n²) bug.
(filesets-ormap): Simplify.
(filesets-some, filesets-member, filesets-sublist): Make them
obsolete aliases.
(filesets-reset-fileset): Simplify.
(filesets-directory-files): Use `push`.
(filesets-spawn-external-viewer): Use `mapconcat` to fix O(n²) bug.
(filesets-cmd-get-args): Use `mapcan` to fix O(n²) bug.
(filesets-run-cmd): Use `mapconcat` and `mapcan` to fix O(n²) bugs.
(filesets-ingroup-collect-finder): Use dynamic scoping.
(filesets-ingroup-collect-files): Use `nreverse` to fix O(n²) bug.
(filesets-ingroup-collect-build-menu): Use `mapcan` to fix O(n²) bug.