* wl-summary.el (wl-summary-buffer-message-ring): New internal

* wl-summary.el (wl-summary-buffer-message-ring): New internal
variable.
(wl-summary-buffer-last-displayed-msg): Abolish.
(wl-summary-push-message): New function.
(wl-summary-pop-message): Ditto.
(wl-summary-goto-folder-subr): Reset
`wl-summary-buffer-message-ring'.
(wl-summary-pop-to-last-message): New command.
(wl-summary-goto-last-displayed-msg): Call
wl-summary-pop-to-last-message if optional argument is non-nil or
the last command is itself.
(wl-summary-redisplay-internal): Push last message number into
`wl-summary-buffer-message-ring'.

* wl-vars.el (wl-summary-message-ring-max): New user option.