Issue 4365: Add ly_is_smob<T>(S) to check if S is a smob of type T. (issue 236850043 by address@hidden)

[nine . fierce . ballads]

Reviewers: dak,

Description:
Add ly_is_smob<T>(S) to check if S is a smob of type T.

Hide Smob_base<>::is_smob(S) to prevent the misleading situation of
D::is_smob(S) returning true when S represents an ancestor of D rather
than a D.  This issue was previously worked around by overriding
is_smob() in several derived smob classes; that is no longer needed.

Hide Smob_base<>::smob_p() because there doesn't seem to be a need for
it to be public.

Please review this at https://codereview.appspot.com/236850043/

Affected files (+141, -161 lines):
  M lily/accidental-placement.cc
  M lily/ambitus-engraver.cc
  M lily/axis-group-engraver.cc
  M lily/axis-group-interface.cc
  M lily/bar-check-iterator.cc
  M lily/beam-collision-engraver.cc
  M lily/beam-quanting.cc
  M lily/book.cc
  M lily/break-alignment-interface.cc
  M lily/break-substitution.cc
  M lily/chord-name-engraver.cc
  M lily/context.cc
  M lily/context-property.cc
  M lily/dots-engraver.cc
  M lily/engraver.cc
  M lily/fingering-engraver.cc
  M lily/function-documentation.cc
  M lily/grob.cc
  M lily/grob-closure.cc
  M lily/grob-info.cc
  M lily/grob-property.cc
  M lily/grob-scheme.cc
  M lily/include/engraver.hh
  M lily/include/global-context.hh
  M lily/include/item.hh
  M lily/include/lily-guile-macros.hh
  M lily/include/music.hh
  M lily/include/paper-score.hh
  M lily/include/performance.hh
  M lily/include/performer.hh
  M lily/include/smobs.hh
  M lily/include/spanner.hh
  M lily/include/stream-event.hh
  M lily/lexer.ll
  M lily/lily-lexer.cc
  M lily/multi-measure-rest.cc
  M lily/music.cc
  M lily/music-scheme.cc
  M lily/new-fingering-engraver.cc
  M lily/note-column.cc
  M lily/note-heads-engraver.cc
  M lily/note-performer.cc
  M lily/note-spacing-engraver.cc
  M lily/page-layout-problem.cc
  M lily/paper-book.cc
  M lily/paper-column.cc
  M lily/paper-column-engraver.cc
  M lily/parser.yy
  M lily/pitched-trill-engraver.cc
  M lily/prob.cc
  M lily/pure-from-neighbor-interface.cc
  M lily/score.cc
  M lily/side-position-interface.cc
  M lily/simple-closure.cc
  M lily/slur.cc
  M lily/spaceable-grob.cc
  M lily/spacing-basic.cc
  M lily/spacing-determine-loose-columns.cc
  M lily/spacing-engraver.cc
  M lily/stem.cc
  M lily/stem-engraver.cc
  M lily/stream-event.cc
  M lily/stream-event-scheme.cc
  M lily/tempo-performer.cc
  M lily/tie.cc
  M lily/tie-engraver.cc
  M lily/tie-performer.cc
  M lily/timing-translator.cc
  M lily/translator-group.cc
  M lily/tuplet-bracket.cc
  M lily/vertical-align-engraver.cc
  M lily/volta-engraver.cc