Automatically unfold percent and tremolo repeats in MIDI output. (issue 769) (issue 40720060)

[k-ohara5a5a]

I think I like the approach.

Right now, we can
\score {
 \music
 \layout{} }
\score {
 \unfoldRepeats \music
 \midi{} }

And if I understand, when this is finished we can
\score {
 \music
 \layout{}
 \midi{midiUnfoldRepeats = #'(percent-repeated-music) } }

I did not before realize that the music is iterated *twice* when there
is a \layout and \midi in the same \score

I think we need the tremolo-type property to hold the 32 from input
c2:32  From what I can see, this type of tremolo will not be repeated
(nor is it with unfoldRepeats)

The other limitation of \unfoldRepeats is that it fails to see repeats
in parallel expressions << {\repeat volta 2 s1 } {c2 d2} >> while this
approach using the iterators will see them and be able to expand them
when requested.

https://codereview.appspot.com/40720060/