Add optional eager black-holing, with new flag -feager-blackholing

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.