guile 2.0?

Now that Guile 2.0 has been out for awhile, anyone mind if we update
cgen to require it?
There are some non-upward-compatible changes I'm seeing in 2.0.9,
and I'm not sure we need to support 1.6 or 1.8 any longer.

I can also imagine taking advantage of some newer features, but at least
for a first step I'd like to get cgen usable with the latest Guile.
[Whether to stick with R*RS or use some Guile-specific features
is a good question, I'm leaning towards R*RS though I always imagined
replacing cos.scm with something like goops.]