reduceComputed dependent keys may refer to @self.

This is mostly useful for array proxies, and in particular array controllers
that use item controllers.  This allows users to, for instance, sort items
according to properties specified on array controllers.

@self is implemented only in reduceComputed because it doesn't really make sense
for non-reduceComputed CPs.

Thanks to @tchak for pointing out the need for this.

[FEATURE reduceComputedSelf]