When the compiler crashes, it is not uncommon to see many hundreds
of goroutines all blocked waiting their turn to be one of the nWorkers
goroutines that is allowed to run. All these goroutine stacks are not a
terribly efficient use of memory, and they also make the crash dumps
hard to read.
Introduce a manager goroutine to hand out work to at most nWorker
goroutines, maintaining pending work in a local slice, rather than
having all those blocked goroutines hanging around waiting to run.