When you use `par` to make a spark, if the spark pool on the current
capability is full then the spark is discarded. This represents a
loss of potential parallelism and it also means there are simply a
lot of sparks around. Both are things that might be of concern to a
programmer when tuning a parallel program that uses par.

The "+RTS -s" stats command now reports overflowed sparks, e.g.
SPARKS: 100001 (15521 converted, 84480 overflowed, 0 dud, 0 GC'd, 0 fizzled)