Add online-help support to describe types

* lisp/help-fns.el (describe-symbol-backends): Move to help-mode.el.
(describe-symbol): Improve the selection of default.

* lisp/help-mode.el: Require cl-lib.
(describe-symbol-backends): Move from help-fns.el.
(help-make-xrefs): Use it.

* lisp/emacs-lisp/cl-extra.el (describe-symbol-backends): Add entry
for types.
(cl--typedef-regexp): New const.
(find-function-regexp-alist): Add entry for types.
(cl-help-type, cl-type-definition): New buttons.
(cl-find-class): New function.
(cl-describe-type): New command.
(cl--describe-class, cl--describe-class-slot)
(cl--describe-class-slots): New functions, moved from eieio-opt.el.

* lisp/emacs-lisp/cl-generic.el (cl--generic-method-documentation)
(cl--generic-all-functions, cl--generic-specializers-apply-to-type-p):
New functions.  Moved from eieio-opt.el.
(cl--generic-class-parents): New function, extracted from
cl--generic-struct-specializers.
(cl--generic-struct-specializers): Use it.

* lisp/emacs-lisp/cl-macs.el (cl-defstruct): Use pcase-dolist.
Improve constructor's docstrings.
(cl-struct-unknown-slot): New error.
(cl-struct-slot-offset): Use it.

* lisp/emacs-lisp/cl-preloaded.el (cl-struct-define): Record the type
definition in current-load-list.

* lisp/emacs-lisp/eieio-core.el (eieio--known-slot-names): New var.
(eieio--add-new-slot): Set it.
(eieio-defclass-internal): Use new name for current-load-list.
(eieio-oref): Add compiler-macro to warn about unknown slots.
* lisp/emacs-lisp/eieio.el (defclass): Update eieio--known-slot-names
as compile-time as well.  Improve constructor docstrings.

* lisp/emacs-lisp/eieio-opt.el (eieio-help-class)
(eieio--help-print-slot, eieio-help-class-slots): Move to cl-extra.el.
(eieio-class-def): Remove button.
(eieio-help-constructor): Use new name for load-history element.
(eieio--specializers-apply-to-class-p, eieio-all-generic-functions)
(eieio-method-documentation): Move to cl-generic.el.
(eieio-display-method-list): Use new names.

* lisp/emacs-lisp/lisp-mode.el (lisp-imenu-generic-expression):
Add "define-linline".
(lisp-fdefs): Remove "defsubst".
(el-fdefs): Add "defsubst", "cl-defsubst", and "define-linline".

* lisp/emacs-lisp/macroexp.el (macroexp--warned): New var.
(macroexp--warn-and-return): Use it to avoid inf-loops.
Add `compile-only' argument.

lisp/emacs-lisp/macroexp.el

@@ -119,20 +119,28 @@ and also to avoid outputting the warning during normal execution."
   (member '(declare-function . byte-compile-macroexpand-declare-function)
           macroexpand-all-environment))
 
+(defvar macroexp--warned (make-hash-table :test #'equal :weakness 'key))
 
-(defun macroexp--warn-and-return (msg form)
+(defun macroexp--warn-and-return (msg form &optional compile-only)
   (let ((when-compiled (lambda () (byte-compile-log-warning msg t))))
     (cond
      ((null msg) form)
      ((macroexp--compiling-p)
-      `(progn
-         (macroexp--funcall-if-compiled ',when-compiled)
-         ,form))
+      (if (gethash form macroexp--warned)
+          ;; Already wrapped this exp with a warning: avoid inf-looping
+          ;; where we keep adding the same warning onto `form' because
+          ;; macroexpand-all gets right back to macroexpanding `form'.
+          form
+        (puthash form form macroexp--warned)
+        `(progn
+           (macroexp--funcall-if-compiled ',when-compiled)
+           ,form)))
      (t
-      (message "%s%s" (if (stringp load-file-name)
-                          (concat (file-relative-name load-file-name) ": ")
-                        "")
-               msg)
+      (unless compile-only
+        (message "%s%s" (if (stringp load-file-name)
+                            (concat (file-relative-name load-file-name) ": ")
+                          "")
+                 msg))
       form))))
 
 (defun macroexp--obsolete-warning (fun obsolescence-data type)
@@ -208,30 +216,30 @@ Assumes the caller has bound `macroexpand-all-environment'."
        (macroexp--cons
         'condition-case
         (macroexp--cons err
-                    (macroexp--cons (macroexp--expand-all body)
-                                (macroexp--all-clauses handlers 1)
-                                (cddr form))
-                    (cdr form))
+                        (macroexp--cons (macroexp--expand-all body)
+                                        (macroexp--all-clauses handlers 1)
+                                        (cddr form))
+                        (cdr form))
         form))
       (`(,(or `defvar `defconst) . ,_) (macroexp--all-forms form 2))
       (`(function ,(and f `(lambda . ,_)))
        (macroexp--cons 'function
-                   (macroexp--cons (macroexp--all-forms f 2)
-                               nil
-                               (cdr form))
-                   form))
+                       (macroexp--cons (macroexp--all-forms f 2)
+                                       nil
+                                       (cdr form))
+                       form))
       (`(,(or `function `quote) . ,_) form)
       (`(,(and fun (or `let `let*)) . ,(or `(,bindings . ,body) dontcare))
        (macroexp--cons fun
-                   (macroexp--cons (macroexp--all-clauses bindings 1)
-                               (macroexp--all-forms body)
-                               (cdr form))
-                   form))
+                       (macroexp--cons (macroexp--all-clauses bindings 1)
+                                       (macroexp--all-forms body)
+                                       (cdr form))
+                       form))
       (`(,(and fun `(lambda . ,_)) . ,args)
        ;; Embedded lambda in function position.
        (macroexp--cons (macroexp--all-forms fun 2)
-                   (macroexp--all-forms args)
-                   form))
+                       (macroexp--all-forms args)
+                       form))
       ;; The following few cases are for normal function calls that
       ;; are known to funcall one of their arguments.  The byte
       ;; compiler has traditionally handled these functions specially