[development] Deletion API hits core

[Barry Jaspan]

Offhand, it looks like this could be very helpful for implementing
foreign keys with CASCADE on delete (the only way we can possibly
implement them).  The trick with doing so is that we have to handle
deletions for modules that are disabled because not all databases
support foreign keys/CASCADE.  Drupal core, or schema.module, or
something could implement hook_delete_alter() to add any necessary
delete statements for those disabled modules.

Of course, this implies that the schema system needs to have the
hook_schema results for all installed but disabled modules.  That
shouldn't be a problem to get.

I still do not think this is doable in time for D6, though.  But it is
very interseting.

Barry