Fix compatibility with Emacs 27

* 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'.

lisp/org-compat.el

@@ -75,12 +75,6 @@
 
 ;;; Emacs < 28.1 compatibility
 
-(if (fboundp 'length>)
-    (defalias 'org-length> #'length>)
-  (defun org-length> (sequence length)
-    "Return non-nil if SEQUENCE is longer than LENGTH."
-    (> (length sequence) length)))
-
 (if (fboundp 'file-name-concat)
     (defalias 'org-file-name-concat #'file-name-concat)
   (defun org-file-name-concat (directory &rest components)

lisp/org-element.el

@@ -7058,6 +7058,7 @@ of FUNC.  Changes to elements made in FUNC will also alter the cache."
                                (inhibit-message t))
                            (condition-case nil
                                (if (and (fboundp 'native-comp-available-p)
+                                        (fboundp 'native-compile)
                                         (native-comp-available-p))
                                    ;; Use native compilation to even better
                                    ;; performance.

lisp/org-persist.el

@@ -110,7 +110,9 @@ dependency means that data shared between variables will be preserved
   (when inherit
     (let ((inherited-index (org-persist--get-index inherit buffer)))
       (unless (memq var (plist-get inherited-index :variable))
-        (push var (plist-get inherited-index :variable)))))
+        (setq inherited-index
+              (plist-put inherited-index :variable
+                         (cons var (plist-get inherited-index :variable)))))))
   (org-persist--get-index var buffer)
   (when buffer
     (add-hook 'kill-buffer-hook #'org-persist-write-all-buffer 1000 'local)))
@@ -128,7 +130,7 @@ When BUFFER is `all', unregister VAR in all buffers."
                                (or (buffer-base-buffer buffer)
                                    buffer))
                               (plist-get plist :path))))
-             (if (length> (plist-get plist :variable) 1)
+             (if (> (length (plist-get plist :variable)) 1)
                  (progn
                    (setq plist
                          (plist-put plist :variable
@@ -137,7 +139,7 @@ When BUFFER is `all', unregister VAR in all buffers."
                    nil)
                (let ((persist-file (org-file-name-concat org-persist-path (plist-get plist :persist-file))))
                  (delete-file persist-file)
-                 (when (directory-empty-p (file-name-directory persist-file))
+                 (when (org-directory-empty-p (file-name-directory persist-file))
                    (delete-directory (file-name-directory persist-file))))
                'delete-from-index)))
          org-persist--index))
@@ -249,7 +251,7 @@ When BUFFER is `all', unregister VAR in all buffers."
             (push index new-index)
           (when (file-exists-p persist-file)
             (delete-file persist-file)
-            (when (directory-empty-p (file-name-directory persist-file))
+            (when (org-directory-empty-p (file-name-directory persist-file))
               (delete-directory (file-name-directory persist-file)))))))
     (setq org-persist--index (nreverse new-index))))