Recentf cleanup optmization

[Vincent Belaïche]

Dear Emacs expert,

I realized that recentf cleanup is very slow when  you have a long list
of saved items (e.g. as few as 60). This is because there is a duplicate
suppression based on linear search (quadratic time).

Starting from latest code on CVS I made an optimization using a hash
table, and this solves the issue.

Please find it herein attached.

Best regards,

   Vincent.

recentf.tgz
Description: Optimization