Radish: decomposed aggregates

[bmyerz]

Should more cleanly define aggregates in terms of

• init: initialize the state
• update/add: add a new input to the state
• combine: combine two states
• emit: transform a state into an output

work in progress on the Grappa side is here https://github.com/uwsampa/grappa/tree/bmyerz/decomposed-aggregates

• See the Chapel paper Global-View Abstractions for User-Defined Reductions and Scans for guiding ideas.

The main thing that needs to change in Raco is that update and combine definitions use MaterializedTuple... as the input/output types, but these tuples cannot handle the custom state types of the Grappa aggregates. Probably the solution is to have the update and combine definitions just use std::tuple directly.

[bmyerz]

480 fixes this for decomposed groupbys;

we also want it for global groupbys per the above designs