Par framework should support nested parallelism

[lars-t-hansen]

Nested parallelism is probably desirable, ie, a worker should be allowed to invoke Par.invoke(), suspending until that subcomputation is done. Such a facility would allow computations to be subdivided recursively.

(Broadcast and eval are in a sense less obvious because they modify global state in the workers. Invoke can do that too, but doesn't have to.)

Recursive invoke() can probably be implemented by having a notion of nested work sets, one work set at each nesting level. A worker might grab a work item from the innermost set, until that set is exhausted, or might grab a work item from a set with some probability that is higher for inner sets and lower for outer sets. Presumably work has been done on this kind of problem before.