guile 2.0.0 out

[Graham Percival]

Guile 2.0.0 is out.

- no, there is no way that we'll try to add this for 2.14.  Nice
  try!  :)
- changing to 2.0.0 could be done immediately after starting 2.15,
  as long as three conditions are met:
    1. the regtests compile
    2. the docs compile
    3. the latest lilydev contains that version of guile

I realize that working on #1 and #2 might uncover bugs in guile
such that it'll only work with guile 2.0.1 or something like that.
I suggest that we make a dev/guile-2.0 branch to work on the
regtests and docs.

I don't see a need to support both guile 1.8.x and guile 2.0.x at
the same time -- as far as I'm concerned, we can drop any
guile-1.x-isms as soon as 2.15 begins.

Cheers,
- Graham