Improve desugaring to generate less temporary variables.
Emit ?: and , for If and Block in expression position.
Mark scala.Predef as an elidable module accessor.

Just because we know it's fine, and it appears so many times
in the output otherwise.
Update checksizes for the last 3 commits.
Merge pull request #868 from sjrd/less-temp-vars-in-desugar

Less temporary variables in desugaring