If all branches in a case block are a lambda, lift the lambda out. In
many cases, this can save creating a closure then evaluating it
immediately, because the function is already applied to the extra
argument.
This happens in particular with IO based code, where the extra argument
is the world token. One place where this transformation has a big effect
is 'evalRef' so the evaluator is now a bit faster (about 20% on the
small benchmark I tried it on - but no guarantees that's going to happen
on other examples!)
