Issue 4594: Rewrite Multi_measure_rest_engraver to improve clarity.

In the process of trying to understand the mmrest engraver, I rewrote lots of it.  It  still works the same according to the regression tests, but I've eliminated some state, factored out some functions, renamed some variables, tweaked some logic, etc.

[Dan Eble, 2015-09-06 13:28:02]

On 2015/09/06 00:03:41, Dan Eble wrote:

To prospective reviewers: I'm sorry this is such a huge patch.  I'll break it
down to make it easier for you.  I expect to post a series of patch sets and
I'll let you know when it's all here.

[Dan Eble, 2015-09-06 18:45:31]

comments, naming, style

[Dan Eble, 2015-09-06 18:46:30]

move most processing into process_music()

[Dan Eble, 2015-09-06 18:47:49]

simplify logic (the interesting part)

[Dan Eble, 2015-09-06 18:48:52]

rearrange for readability

[Dan Eble, 2015-09-06 18:51:55]

Reviewers, I tried to concentrate the real changes in patch set 3.  TIA.