Partial Reductions

[bradcray]

As a Chapel Programmer, I want to have access to partial reductions because they support a common and important computational idiom while also providing opportunities for the Chapel implementation to optimize the computation better than a typical programmer would be able or willing to do.

Acceptance criteria:
Partial reductions are designed, documented, and implemented.

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The implementation of partial reductions is expected to change the user-defined reduction and domain map interfaces.