Introduced the function
parallelCheck :: [Check a] -> Check [a]
that runs independent checks in parallel, potentially allowing faster grammar
compilation on multi-core computers, if you run gf with +RTS -N.
However, on my dual core laptop, this seems to slow down compilation somewhat
even though CPU utilization goes up as high as 170% at times.
(This is with GF compiled with GHC 7.0.4.)