names for enabling full measure rests

[Graham Percival]

I'm not totally certain about these names, so I thought I'd ask for
other recommendations.

expandRestBars = {
  \set Score.skipBars = ##f
}
contractRestBars = {
  \set Score.skipBars = ##t
}

\expandRestBars isn't particularly useful, but since we're adding
\contractRestBars I figured we might as well add the other one as
well.  We're adding these because it's ridiculous to have a really
common, useful feature like full measure rests which require an explicit
\set command.

Cheers,
- Graham