WIP: support combiners for UDAs

- [x] Associate an optional decomposable state with UDAs; this guides the decomposition into local, remote aggregates
- [x] Expand decomposable UDAs in a new optimizer rule
- [x] Test a simple sum UDA using this new framework
- [x] Plumb syntax for specifying combiners into the Myrial language
- [x] More tests!
- [x] Clean up redundancy between built-in aggregates, UDAs

Sanity tests appreciated.
why overload the definition in `AggregateExpression`?