[D4] Small bug in #publishedAspects

The expression:

    #(1 2 3) asSet publishedAspects.

gives a walkback complaining that Set isn't indexable -- the reason
seems to be that the specialised implementation of #publishedAspects, in
Collection, uses self #publishedKeyedAspects, but the default
implementation of that (not overridden by Set) is just:

    ^self errorNotKeyed

I suspect the default implementation should be something like:

    ^ LookupTable new.

    -- chris