* lisp/org-compat.el: Remove `length>' alias
* lisp/org-element.el (org-element-cache-map): Suppress compiler
warning.
* lisp/org-persist.el (org-persist-register): Do not `push' into
`plist-get'. Not supported yet in Emacs 27.
(org-persist-unregister): Do not use `length>'. Use
`org-directory-empty-p'.
(org-persist-gc): Use `org-directory-empty-p'.