IndexableCollection created, HashedCollection reparented.
SequenceableCollection reparented, by hand.
Comments for {Indexable,Sequencable}Collection.
IndexableCollection >> {with:do:,withIndexDo:}
IndexableCollection >> at: family
IndexableCollection >> indexOf:{,ifAbsent:}

Also removed strange `keyAtValue:` which no one used
and was invalid (detect with two-arg block, reacting on 2nd arg).
Merge pull request #388 from herby/indexable-collection

Indexable collection