Implementing silence consolidation.
Improved meter rewriting in RhythmManager.
Added MusicSpecifier.is_sentinel property.

When true, `is_sentinel` indicates that the music specifier represents silence
only. This makes calls to inspect_(music).get_effective(MusicSpecifier)
simpler.
Improved rest consolidation in RhythmManager.
Rerendered all segments with silence consolidation.
Wrote timespan-related segments 7 and 8.