* scm/grob-description.scm (Beam): Add concaveness. Replace
Beam::cancel_suspect_slope with Beam::check_concave.
* lily/beam.cc (check_concave): Calculate concaveness of beam, and
set slope to horizontal if concaveness > Beam.concaveness. This
handles cases that kludgy cancel_suspect_slope was meant to catch
very well.
(cancel_suspect_slope): Remove.