Creates Skyline_forest smob (issue 7098069)

[dak]

https://codereview.appspot.com/7098069/diff/1/lily/skyline-forest.cc
File lily/skyline-forest.cc (right):

https://codereview.appspot.com/7098069/diff/1/lily/skyline-forest.cc#newcode2
lily/skyline-forest.cc:2: This file is part of LilyPond, the GNU music
typesetter.
And the license makes 95% of all the comments in this file adding new
functionality.  There is no comment either here or in the  .hh file that
explains what use case this code is designed for, why it is necessary,
and how it does the job it has been written for.

There is no definition of a "skyline forest", what differentiates it
from a skyline, what operations it does provide for which use cases
under which cost and considerations.

There is no why, what, and how explained anywhere.  It is just a dump of
code without any rationale or explanation.

This code does not even try to be comprehensible to or maintainable by
anybody but Mike, so it should not leave the repository of Mike in this
state.

https://codereview.appspot.com/7098069/