Issue 4423: eliminate part combiner's array of context handles (4/4)

Issue 4423: eliminate part combiner's array of context handles (4/4)

The Part_combine_iterator no longer creates or keeps alive the Voice
contexts that it uses.  make-directed-part-combine-music does it.

When the Part_combine_iterator needs to redirect a child iterator, it
finds the new Voice by name among the siblings of the current Voice.