Beautify Grob_array and stop using std::vector::data() (issue 264950043 by address@hidden)

[nine . fierce . ballads]

Reviewers: ,

Description:
Beautify Grob_array and stop using std::vector::data()

Remove the nasty workaround for platforms that don't provide
std::vector::data().

Replace most code that reached into into Grob_array and modified its
internal vector with calls to new member functions such as filter().

Move the definitions of some trivial methods into grob-array.hh.

Please review this at https://codereview.appspot.com/264950043/

Affected files (+74, -147 lines):
  M flower/include/std-vector.hh
  M lily/break-substitution.cc
  M lily/grob-array.cc
  M lily/include/grob-array.hh
  M lily/paper-column.cc
  M lily/pure-from-neighbor-interface.cc
  M lily/slur.cc
  M lily/system.cc