Add optional eager black-holing, with new flag -feager-blackholing
Eager blackholing can improve parallel performance by reducing the chances that two threads perform the same computation. However, it has a cost: one extra memory write per thunk entry. To get the best results, any code which may be executed in parallel should be compiled with eager blackholing turned on. But since there's a cost for sequential code, we make it optional and turn it on for the parallel package only. It might be a good idea to compile applications (or modules) with parallel code in with -feager-blackholing. ToDo: document -feager-blackholing.