[development] Deletion API hits core

[Chad Phillips -- Apartment Lines]

well, after three development cycles, i've apparently written  
something worth committing for Drupal's deletion system...  ;)

thanks to all the folks who helped it along, esp. chx and steven --  
couldn't have done it w/o you!

now, on to other matters:

upgrade docs are done, and here:  http://drupal.org/node/ 
114774#deletion-api

a complete writeup on the API is here: http://drupal.org/node/153904

hook_delete_pre() and hook_delete_alter() have been added to the  
developer docs in contrib.

the new functions are on api.drupal.org in HEAD.  their formatting is  
a bit crappy, and i'm not sure if that's an api module bug or  
something i did.  at any rate they're usable, and i'll look into  
getting them prettier in a bit.  :)

currently, only node and comment deletions use the API in core.  we  
have about 8 days until the code freeze, so i wanted to get opinions  
on what else in core we might want to try and convert.  IMO, the one  
that should definitely get converted is user deletions, which i'm  
happy to write.  anybody else have thoughts on other important areas  
that we should focus on before the code freeze??