Issue 4423: Part_combine_iterator: eliminate array of context handles

This review covers multiple commits.

[1] Part_combine_iterator: move mmrest killing after child iteration

Also limit mmrest killing to contexts that have just changed from
being used to being unused.

[2] Part_combine_iterator: create child iterators in parent context

Add an initial split state to ensure that the first split-list entry,
whatever it is, will redirect the outlets for the child iterators.

[3] Part_combine_iterator: use NullVoice instead of Devnull

This makes all outlets a kind of Voice.

[4] Part_combine_iterator: eliminate array of context handles

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.

[Dan Eble, 2015-05-28 11:47:02]

Here's the next battle in the war against the Part_combine_iterator.
I haven't created a Google Code issue yet.

[pkx166h, 2015-05-28 13:02:54]

On 2015/05/28 11:47:02, Dan Eble wrote:
> Here's the next battle in the war against the Part_combine_iterator.
> I haven't created a Google Code issue yet.

https://code.google.com/p/lilypond/issues/detail?id=4423