(flyspell-last-buffer): New var.

(flyspell-last-buffer): New var.
(flyspell-accept-buffer-local-defs): Use it to avoid silly redundant work.
(flyspell-mode-on): Use add-hook for after-change-functions.
(flyspell-mode-off): Use remove-hook for after-change-functions.
(flyspell-changes): Make it buffer-local.
(flyspell-after-change-function): Make it non-interactive.  Use push.
(flyspell-post-command-hook): Check input-pending-p while processing
the potentially long list of buffer changes.