Issue 4594: Rewrite Multi_measure_rest_engraver to improve clarity. (issue 268740044 by address@hidden)

[nine . fierce . ballads]

Reviewers: ,

Description:
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.

Please review this at https://codereview.appspot.com/268740044/

Affected files (+142, -155 lines):
  M lily/multi-measure-rest-engraver.cc