Accumulator objects

[lars-t-hansen]

Sawzall has a notion of output objects, which are accumulators that workers can emit into in a safe and predictable way: http://szl.googlecode.com/svn/doc/sawzall-spec.html#output_types.

Rather than simple counting accumulators (see Issue #18) these can process the data as they are accumulated. They can probably be built from either critical sections or multi-value atomic objects, depending on what they're doing. They probably require a real allocator (see Issue #19). They can be implemented in user code but seem generally useful and providing a couple of them would be good.