Issue 5607: typedef System::rank_type

https://sourceforge.net/p/testlilyissues/issues/5607/

1. break-substitution: typedef System_range to clarify code

Call an interval of System rank a "System_range" rather than a "Slice."

Though both of these are currently Interval_t<int>, there is code that
stores System ranks in shorts.  At some point, it might make sense to
change System_range to Interval_t<short> for consistency.

2. typedef System::rank_type

Code in break-substitution.cc previously assumed that there would not
be more than 32k systems.  This centralizes that assumption in System
and checks for programming errors.

[Dan Eble, 2019-11-19 17:37:48]

typedef System::rank_type

[Dan Eble, 2019-11-19 17:50:00]

https://codereview.appspot.com/581270043/diff/547260043/lily/break-substituti...
File lily/break-substitution.cc (right):

https://codereview.appspot.com/581270043/diff/547260043/lily/break-substituti...
lily/break-substitution.cc:211: vector<Spanner *> const &bs = sp->broken_intos_;
I hereby dedicate this non-cosmetic change to Werner.  If I had simply replaced
"Slice" with "System_range", the code below would have exceeded 80 columns.

[lemzwerg, 2019-11-19 21:49:36]

:-)