lily-guile updates and CG: "Scheme->C interface" section. (issue 4917044)

[bordage . bertrand]

Reviewers: ,

Message:
Hi,

Graham asked me to document an small issue.
(http://codereview.appspot.com/4875054/)
And this became something bigger.

He also told me to push it directly, but I made a few changes in the
interface as I read the code and wrote the doc.
So the review concerns only the C files of this patch, but feel free to
tell me if I was totally wrong when writing it.

Thanks,
Bertrand

Description:
lily-guile updates and CG: "Scheme->C interface" section.

Please review this at http://codereview.appspot.com/4917044/

Affected files:
  M Documentation/contributor/programming-work.itexi
  M lily/general-scheme.cc
  M lily/include/lily-guile.hh
  M lily/lily-guile.cc